People complaining about this being a remake, but I'm assuming they made the remake as a way for their developers to get use to Unreal 5 before making Witcher 4 which is also built using Unreal 5.
Easier to build a game you already know in a new game engine than to create a completely new game. Especially if you don't want the game to be filled with bugs.
CD has always used in an house game engine for their games.