I don't want to wade into it too far. But it also could be a bit of Sancho wanting to show United they screwed it up. He might be putting in more effort in training, practice, whatever, so it wouldn't be unreasonable to expect some uptick in performance. It doesn't always go that way, but he's been successful before at Dortmund.
I don't want to get into ETH too far beyond, I'm fine if he stays for as as he likes. He's a symptom of general problems at the club, but he's also likely screwed it up too far to where I don't know if he could bring it back from where it is. Neither Klopp or Pep would have likely bought Sancho. But either one of them could have gotten more out of him. How much the alienation is attempting to play his guys, establish routines in the squad, or whatever... I don't really know. I think there's ample blame to go around, along with a squad that has pretty much run off or quit on other managers.