As the first one to train Kenneth's Snatch Vo2 protocols after his presentation at the level 2 RKC back in 2007 I've gone back and forth with him over the years as to whether swings would be a suitable substitute for snatches in these protocols. His answer was that that were ok but not as good as the snatches because "with swings there is too much continuous tension" as opposed to the snatches where there is quite a bit of 'unloading' of tension ( and therefore the heart) during the float phases of the ascent and descent.
This unloading of tension was what makes the snatch unique as the closest one can get with resistance tools to actually cardiovascular training which always includes a relaxation phase as part of the cyclical nature of classic cardio/aerobic exercise ( i.e running, swimming, biking).
The reason ' lifting weights fast' is not true 'cardio' is just that- too much continual tension on the heart.
Will swings improve your conditioning and your hearts ability to go hard for a long period of time? Undoubtedly. Is it "true" aerobic exercise? No. But that's relatively easy to get with just fast walking ,rucking etc.
The one thing that is usually missed about KJ's max vo2 snatch training is that the goal was always to train your HR up to actually max levels ( I usually got to 185-200) during work sets. the reason this was possible is because of the 15 second breaks every 15 seconds where the HR can come down enough to allow you to continue, only to have it pushed back up after another 15 seconds of work.
I'm not sure if my max vo2 actually improved over the years I trained it but my resting hr did drop from 62 to 48 in the mornings after the first 8 weeks of training. And, over the course of a year I worked up from 35 sets of 7 with a 16 kg bell to 40 sets of 8 with the same bell. That is one very serious workout from any perspective.
this was my last workout before I did 80 sets of 8
BTW my training for the 48 kg sinister swing test ( which I accomplished although I didn't swing the bell to standard height enough reps) was much much harder just given the sheer mass of the bell.Not sure I was in better "shape" though than I was snatching the 16