Sounds like a bit of a circular argument, Steve. If playing in a particular way is proof to you that people have learnt "from the dots" then that proves that learning from the dots causes them to play that way. Do you have separate evidence of how people have learnt their tunes? Did you ask them?

You made that circular argument, not me. I've played in sessions every week for 20 years almost without fail. If someone turns up with a tune they've learned from dots, or from a single recorded source, it's bleedin' obvious. They will play it their way and no other way. They fail to see how vitally important it is to accommodate other people's take on the tunes. And it puts them in grave danger of not listening flexibly to the other musicians.