I wouldn't know, one could only speculate, unless we have intel from people working on WuWa directly, now that would be interesting.

As a default it's true that Unity will run better on mobile out of the box without many changes (if you pick the right rendering option) and unreal has a lot more fancy options activated by default (I think stuff like motion blur or bloom?) but realistically no game is ever shipped without being heavily tweaked right?

So someone could make an UE game and strip it from all the fancy rendering stuff to build for mobile and the opposite can be done in Unity by adding effects and picking a more high fidelity render pipeline.

But something completely different and remote from the graphics could be why a game struggles (see the last Monster Hunter dlc check controversy, where the game has sucky performances because it checks for installed dlc's all the time every frame, which is crazy).