Personally, I think this game should have been made with Ren'py. It's not a big enough game or rich asset heavy. Just Koikatsu, which doesn't really need more to be run except Unity or Ren'py.
Renpy is easy to pick up for someone with no prior coding knowledge but is pretty limited in what it can do compared to other engines.
And the coding knowledge from renpy (python code) doesn't really transfer to other engines. If I'm not mistaken unreal and unity use C# which is quite different from python.
And if I remember correctly Bewolftraize said a while ago that he wanted to learn to make games with Unreal engine so he can:
-improve the game (further than he could with renpy) as he learns.
-make better games in the future.
And regarding the visuals of the game, he's using Koikatsu but it would be the same if he was using Daz3D, Honey Select, Blender, etc.
As long as he doesn't make a 3D game (1st/3rd person with wasd movement) all the visuals will be created and rendered in those softwares and turned into image and video files that are read by the game engine.