That can be true for any commodity software though. Designing something inhouse means you inherently will have engineers and experts with better low level understanding. It doesnt mean it will be better (could even be much worse) but theres a tradeoff there.
So a lot of studios think oh, the engine is already finished, we just need artists and designers to use it, all the heavy lifting has already been done. And while yes, you can make a game that way, the results will be sub optimal. But then again, free is infinitely cheaper than having an engine team maintaing your own engine or any custom elements of UE.