Im not sure that the protocol used during this experiment counts as varying exercises the way the OP meant it.
Okay, let break down what Rdodo post and provide some "Surety" for you. Not being sure equates to guessing, making a choice and hoping you guess right.
If it's beneficial to vary volume and intensity day to day/week to week, then why not completely vary exercises? (Like Crossfit WODs, for instance)
CrossFit Workout of The Day
This is General Physical Preparedness, GPP, Training which is a "Jack of All Trade, Master of None" approach.
It is somewhat effective for GPP but that it.
Constantly changing exercise on a daily basis is a chaotic approach that yield very little for novice lifter, which Rdodo is. It isn't very effective for advance athletes in sport training, either.
It amount to working a different job each day. You gain some overall knowledge about each job but you will never become proficient.
Thus, Rdodo progress will be less that it could be. But at least he is using his head and trying to think through it.
Muscle Confusion
This is the Bro term used by many Neanderthals.
Muscle Confusion is based on varying exercise as a means of producing increases in strength and size.
CrossFit take it too far with their Workout of The Day.
Muscle Confusion is based on...
The General Adaptation Syndrome
I have posted information on this site into a state of ad nauseam.
The General Adaptation Syndrome is composed of three part.
When presented with stress or trauma (Covid, Strength Training), etc., you either "Adapt or Die".
The issue of adaptation with Strength Training (Limit Strength, Power. Speed, Hypertrophy, Strength Endurance, etc) is that once adaptation occurs, progress stops.
Continuing to push past the adaption point leads to OverReaching and eventually OverTraining; a downward spiral.
Periodization Training
This method is built on The General Adaptation Syndrome.
Planned easy training, with progressive increases in loading/intensity each week. The final week is pushed to the limit.
Once the final week is completed, a New Periodization Training Program is begun with light load and some New Exercises.
This bring us to...
Varying Exercises
Varying Exercises elicits The General Adaptation Syndrome.
The New Exercise initially provide a new stimulus of trauma and stress to the body.
The initial response of the body is to overcome it and become stronger. Thus, there is some validity to, "What doesn't kill you make you stronger".
Training Age
This refers to how long an individual has been training.
1) Novice Lifters
These individual have a longer adaptation time periods.
Performing the same exercises for 8-12 weeks, usually works for them before they need to make changes.
2) Advance Lifters
These individual adapt quickly. They need to make changes every 3 - 4 weeks.
The Westside Powerlifting Method
This method has been around since, circa 1982.
It is a very successful method.
One of the fundamental of it is to Vary Auxiliary Exercises (Change Strength Exercises) as a means of increasing Limit Strength in the Powerlifts with each New Training Cycle.
Dr. Tom McLaughlin, PhD Biomechanics
McLaughlin, former Powerlifter, did some seminal work on the Biomechanics of the Powerlifter.
His work support The Westside Training Protocol of building strength in each of the Powerlifts with Auxiliary Exercise. The Competition Powerlifts are not performed for repetitions, as The Training Exercise.
Performing a variety of exercises that are similar in nature to the Competition Lifts.
The issue with performing the Competition Lifts as the Training Exercise is it ensures poor technique is developed.
I'll stick with the Strongfirst research.
StrongFirst Researh
StrongFirst does not have a research department.
So, what Research are you talking about?
They do have some good and interesting article.
However, some of those articles contain misinformation.
One of those article is based an authors opinion, which is incorrect. Research and anecdotal data don't support his believe.
And as someone once said, "You feeling don't matter, he Facts do."
Another article presented by one of my favorite people, is based in misinformation that continues to be perpetuated. I provided him with a reputable research article that corrected that information.
However, that same article is still on the site; no correction revision was made.
Thus, the perpetuation of this misinformation continues.
the ones who have the most to gain or lose, are the best lab rats and experiment designers when it comes to strength,
"Everything Works But Nothing Works Forever"
I have noted this in pervious post. A Novice Lifter is going to make the greatest amount of gains, regardless of what they do.
Making progress after that require making changes.
More than anything, strength is a skill, and mastering skills requires hours in the lab drilling into the same movement over and over and over while tweaking and honing every detail.
Technique Development
I have several post on this site regarding Technique Training.
The issue is that many individual incorrectly utilize the movement they are trying to develop skill in as the Training Exercise.
While some strength is gained, it come at the expense of Technique Development.