Not sure how it "works" - i'll leave that to the medical professionals, trainers etc. on here but speaking purely anecdotal here....I appointed a personal trainer and also joined crossfit in my mid to late 20's as my then wife (no ex-wife) had the trainer when we got married. I was always fairly "fast" and have always been quite skinny/scrawny most of life.
Working with the PT and doign crossfit - for the first time in my life i built quite a bit of muscle (and a lot of lean muscle) and was extremely fit. We worked a lot on explosive power and i can tell you my acceleration over 30-40 meters improved drastically. But this was obviously "me" - someone very skinny, who never really gym'd in his life, all of a sudden picking up a decent amount of muscle mass...
Now I know these guys are all professional athletes and their bodies are already extremely finely tuned - almost at their "peak/best", but I really can't think that it won't be possible for Mainoo to improve his acceleration/speed?
Whether it's suddenly going to turn him into a Baleba type pace merchant - is obviously wishful thinking, but I'm sure he can improve. I guess where my uninformed brain gets stuck is that with all our supposed state of the art facilities and medical specialists....this is surely top of mind, so who knows - maybe I'm wrong and it can't really be altered to a significant enough degree to make a difference on the pitch.
@Pogue Mahone maybe you and a few others could comment.