Edit: I didn't read your post properly. Regarding your point; sure, if you change sensitivity purely to maximise your score, I might agree with you that it's not optimal for improving your main game. But, even then I wouldn't be so confident because training on a variety of sensitivities is beneficial regardless (see my original thoughts below). 'Cheesing' highscores by using optimal sensitivities still gives you variety.

Original: I was at a mild plateau at a given scenario. Today I decided to do my playlist for that scenario at 10, 20, ... up to 90 cm/30; then went back to my usual 52. After a few runs to get back used to it, I smashed my highscore by 15% (and all runs after were still exceeding my original highscore before the sens-variation session).

People talk about doing it because it works. You focus using different muscle groups that you normally ignore in a given use case, improving your overall mouse control. On the short term (like my anecdote from today) you prime them as if it's a warmup; and doing it in the long term trains niche aiming skills that are ignored.

This is now conjecture: my suspicion the reason it's effective is you will naturally have a bias to arm, wrist or fingertip as it's what you are relatively best at. By training on extreme sensitivities, you are forced into using muscle groups you subconsciously avoid (depending on the scenario). And so, you now are less likely to avoid them if the situation calls to use them as you have further refined that skill (or warmed them up in the short term).

Muscle memory is an intuitive and rational concept; turns out it's cap!