I don't think he lacks athleticism. I think he lacks in other departments, mostly mentally. The guy simply doesn't have the mental tools (maybe yet, I doubt it'll change, though) to work out at a topclub or a club with the ambition to be one. Dortmund manage to make guys like him or Dembele look good, because their whole business model is reliant on selling these players. So they bend over backwards in order to avoid any scandals or public criticism about their disciplinary issues. Which is why they had a guy hired to wake Sancho in the morning. They are basically enabling their lack of professionalism. Which works out for them, or used to.
Once he or Dembele arrived at a topclub where this shit only flies when you have Messi or Ronaldinho kind of talent, they falter. They are used to having everything, and I mean absolutely everything, going in their interests. They are the biggest fish in a small pond and won't have it any other way.
At United, Barca, Bayern, Real and so on, you only work out if you are professional enough to fight for your spot in every single practice and match. Something Sancho hasn't done once in his life, because his talent was always big enough for him to get a place despite his lack of professionalism. He just never once had to be a professional athlete before. So of course he doesn't know how to be one. And as long as clubs want him and pay him obscene money, as long as his false friends tell him how great he is, he won't understand that he needs to change. Because everything is working out.
But now that talent alone isn't enough anymore, he lacks the mental tools to change his approach. Which is why it made sense (from his flawed perspective) to rejoin a club where he could once again play by virtue of his talent alone.
It is quite similar to gifted children in school. They often tend to be very good early on, but become increasingly worse in the academic world, because they have never learned to learn.