That's debatable, but even if it's correct, being a grafting hard nut isn't what is going to put Amad Diallo on United or any other team. Physicality isn't his game, it's just something he has to learn to deal with to create the space for what is his game. And I question that the optimal way to develop that game is putting him a league and on a team where he has to have a lot of focus on dealing with how physically challenging things are. The PL naturally wouldn't be any better - if he was ready to play there regularly, he wouldn't be loaned out.
But of course, it depends on where you think he is in his development process. If you consider him basically fully formed in his technical game, then he'd be at the point where learning to thrive in this environment should indeed be next on the agenda. If United thinks this is the case, Sunderland makes sense, and so did Rangers before that. But, and this is the big issue for me, he has never at any point in his career started regularly for any senior level team. Ever. He had just a couple of sub appearances for Atalanta, not a lot more for United, and he was an infrequent starter at Rangers. How can anyone be considered more or less fully formed given that? And if he isn't, I don't understand how it's not best for him to go somewhere he can play regularly and focus on the things that are going to be the basis for his game, rather than on the things that are going to work against him. THEN he can go play for Stoke or whatever to learn to deal with the physicality.