I'm pretty sure UE3 is much less optimized for the Switch than UE4. And as for being handled by a third party... so is DOOM Eternal.
Well, Doom Eternal is id Tech 7, not UE4, it's an all new engine that debuting with this game. Still, it's an iterative upgrade over id tech 6 which is well established on Switch with Doom 2016, Wolfenstein II, and Youngblood all running on it. I don't think the engine should be an problem.
Bet with Liquidlaser: I say PS5 and Xbox Series will sell more than 56 million combined by the end of 2023.