His improvement has everything to do with the system. The good stuff he is doing currently is not something he failed at under Xavi, it's something he wasn't instructed to do at all, something he couldn't do when having to stay out wide.
He is still not great at creating with the ball at his feet against a stable defense but he's barely doing that under Flick. Under Xavi generally he got the ball too early in the moves for him to utilize his abilities.
Now he's on the move. In and out of spaces all across the attack, running deep, pressing like a mad man. Getting on the ball when the game is moving, at points when defenses are unsettled, when there is spaces and opportunities.