I may be able to expound on the biochemistry/physiology of heart rate training another time, but first and foremost I'd like to clarify a few things that may have been misread or misunderstood about what I was trying to say.
But before that, allow me to emphasize that a lot of this stuff depends on the individual. Their training history, their healthy history, their diet, their sleep, how they recover, etc. Generalized prescriptions usually become problematic because they miss key pieces of this.
A second point I want to emphasize too, is that how one applies heart rate training, or any training really, is highly dependent on the individual's goals. Are you training for bigger muscles? For weight/fat loss? Are you training for a competition? What are the parameters for doing well at that competition (powerlifting vs. half-marathon race)? Are you training for health and longevity? It's important to define one's goals because you simply can't have it all. If you want to be competitive in a half-marathon, you're going to have to train to be fast. However, cardio training at high intensities for long durations necessitates hypertrophy of the left ventricle of the heart. We see a not insignificant number of endurance athletes develop A-fib, LVH, need pacemakers or even have massive heart attacks on the track because of this physiologic response. In this case, your training goal to be fast & competitive at a 13.1 miles may supersede your goal to be healthy or to live a long life. It's important that your training not only support your primary goal, but that one be aware of areas where the training for a primary goal and the training for a secondary goal may be in conflict.
So anyway, back to my original point from my previous post. HIIT is not always a bad thing, when used appropriately. Getting your heart rate up and using those glycolytic pathways can be beneficial (As previously described, literature has shown increase in EPOC, insulin sensitivity and fat loss with HIIT training). However, for the health & fitness goals that most people have (feel good, look good naked), the emphasis should be on the ratio of this aerobic/below lactate-threshold work volume to anaerobic/glycolytic work volume.
So, in the context of an endurance-biased athlete who is running/jogging say, 360 minutes a week at a heart rate below lactate threshold, 10 minutes of KB swings 6 days a week (weekly total of 60 minutes) is only spending about 16.7% of their total training volume above lactate threshold (as measured by heart rate. Let's note here that lactate threshold only roughly correlates with a specific heart rate, which is subject to the individual in question and their prior training. That is, the heart rate at which one can work at while staying below lactate threshold can and should increase with fitness. Lactate threshold however, is technically a measure blood lactate levels, but since its pretty impractical to stick a needle in one's arm and measure blood lactate throughout a set of KB swings, we are using heart rate for a proxy measure of blood lactate). And that's assuming that they are staying above that lactate threshold heart rate for the entire ten minutes of kettlebells swings, including rest periods. If we parse this out further, to only include the actual time swinging with the heart rate above the approximate lactate threshold, we could safely estimate that the percentage of total training volume above lactate threshold is at or below 10%.
However, if this athlete were to only perform the S&S program, those numbers obviously do a big flip-flop (Maybe not a complete flip flop, but the time above lactate threshold significantly increases compared to ~10%). Which is why if it is the only training one is doing, it would be beneficial to do it as aerobically as possible.
In the context of the OP's initial question, he seemed to indicate that he didn't have the time to devote to performing S&S in a purely aerobic fashion, so my suggestion was to mimic the first scenario above- let his S&S routine be more or less his little bout of HIIT, or <10% of training volume/activity and then find other ways to increase the volume of purely aerobic work- walking to and from work or employing the use of a tread desk at work to capitalize on time in order to include that greater volume of aerobic work. What I was definitely NOT advocating for is that his S&S routine should be done HIIT-style 6 days a week as his only training.
In regards to our evolutionary history, I may have simplified, but I don't believe I 'romanticized.' We only have to look at our current physiology to see what conferred reproductive fitness on our ancestors. Physiologically speaking, we can perform large volumes of work over long periods of time aerobically- walking, easy jogging, light hiking. Aerobically, we can produce 38 molecules of ATP through oxidative phosphorylation (ATP being the energy currency of the cell) and we can also use glucose, fatty acids and amino acids as substrates for this. Anaerobically, we can produce 2 ATP through lactic acid fermentation. Furthermore, the build up of H+ ion from this reaction inhibits phosphofructokinase, effectively shutting off the glycolytic pathway in order to prevent metabolic acidosis, which would lead to death. However, as metabolically inefficient as lactic acid fermentation is, it can produce ATP for muscle contraction MUCH more quickly than the lengthy oxidative phosphorylation process. So, for our earliest ancestors (and these biochemical pathways go back further than just primates and hominids), being able to sprint away from danger kept you alive to reproduce another day. The metabolic inefficiency of it though and the fact that the acid build up would kill you, meant it could only be endured for a short period of time (10-20 second max) and the needed recovery period from the increased cortisol and adrenaline levels meant that is was for emergency situations only. (High cortisol levels can negatively impact fertility by depleting progesterone levels in women and testosterone levels in men.)
So if aerobic metabolism has so many benefits- more ATP, greater volume of work that can be performed at lower intensities, larger variety of substrates that can be used, more favorable hormone profile, etc., why bother to train anaerobic pathways with HIIT training at all? Again, I think this makes sense in light of evolutionary biology. Any exercise or training is a physical stress. With sufficient rest, the body recovers and becomes stronger, in order to be able to better handle that stressor in the future. Going back to our sprinting ancestor, the faster/stronger one survived- and reproduced. The slower/weaker one did not. But how did that individual get to be the faster one? Adaptations like greater muscle strength (specifically hypertrophy or greater number of muscles fibers), greater power via increased motor pool recruitment, etc. As those physiological strategies enabled greater survival, they become more widely incorporated in the physiology of the population. So the ancestor who sprinted from danger and then adapted by getting stronger or burning more fat for energy is the one who got to pass their DNA along to the next generation and that's why we see individuals express those adaptations to HIIT training in the exercise physiology experiments we conduct in the lab.
So again, I don't think I'm romanticizing the idea that based on evolutionary history and biology, for health and longevity purposes as well as reproductive fitness and attractiveness purposes, the optimal strategy is simply a high volume of low intensity work- walking, jogging, hiking, playing with your dog, aerobic KB swings with lots of rest in between sets mixed with little bouts (low volume!) of high-intensity work occasionally ('occasionally ' depends on a number of individual variables). If anything, I was trying to simplify my argument without the having to go into introductory biochemistry, which I totally love to geek out on, but realize, like an Islay Scotch, is a finely acquired taste.