Even if you felt you were fundamentally right and the manager was scapegoating you, you'd just apologise so you could train and play again, after all, surely that's the dream no matter what else is going on and you wouldn't want to let your teammates down because you can't rebuild that trust once it's gone. Sancho isn't stupid, he surely knows Ten Hag won't last the season even if Sancho is selected every week so he's surely doing all this because he wants out of the club.
From what we understand, Sancho and Ten Hag sat down and went through the videos of training sessions and Ten Hag pointed out the points where he felt he wasn't training up to standard and Sancho disagreed with his observations and comments from the training videos. Where do you go from there? You've got a player who believes he knows more than the manager.