This is something that often happens, when a coach moves clubs, he might bring some players from the old club because the player's liking for the game is a consideration, especially if the club is able to bring in the players requested by their new coach, so Enzo saw that there was the opportunity for Kiernan Dewsbury-Hall to become a starter at Chelsea and make Chelsea's game better.
Kiernan's presence will give Enzo more options in midfield and players who do not appear consistently will be replaced by Kiernan who performed quite well at Leicester last season
Yes every manager has who they most trust most especially in the sensitive departments like the midfield, I could remember the likes of Pep guardiol who doesn’t do this often moving Thiago Alcantara from the Barcelona team to his then Bayern Munich team after promoting him then, both of them settled and had success at Bayern Munich together. Jose Mourinho is another manager who loves to reunite with his ex players at a new club most especially the case of Matic who we all know moved to different clubs with him. We have Ancelloti again with Jame Rodriguez and also the love affair between Alex Ferguson with Paul Scholes who had to even come out from retirement for him.
So there is nothing wrong for a manager to get his old player into a new club, but what is wrong is whether the player is much needed at the club. Dewsbury-Hall Is a more of central midfielder rather than an attacking midfielder which to me Chelsea needs badly. They currently have multiple players at this central position (box to box) with players like Gallagher, Enzo Fernandez and even Caicedo players that prefer that position, so adding Dewsbury-Hall to that number doesn’t seems right to me and more like a waste of money at the moment