The biggest share of Unity is 2D mobile games, something Unreal is not particularly suited for and I very much doubted that segment of the market will switch to Unreal.
Unless a very major rewrite happened in the last few years, Defold absolutely does not power Candy Crush, in fact to my knowledge King has never shipped anything in Defold and it's fallen almost entirely out of favor internally.
All the Candy Crushes as well as several other games use variants of a custom internal engine that's managed by a fairly sizable central team.
Ah, you are correct about Candy Crush. After being promoted by your comment to investigate further, I see that King seems to only ever have released one game using the engine: Blossom Blast Saga in 2016, at least according to the showcase[1].
It seems that Candy Crush predates the engine by a few years and that the engine was originally created as a hobby project by one of the King employees, although it’s seen commercial use on mobile by a number of studios and has received backing from King and others over the years. King are not currently listed as a backer of the Defold Foundation, but one of the board members is the “director of engines” at King.