Well yeah, he's working with a pretty small team, and quite successfully: https://en.wikipedia.org/wiki/Celeste_(video_game)

I would say that one of the "Miscellaneous Thoughts" at the end of your article answers your question pretty well:

> I need only the best fancy tech to pull off my game idea

Then use Unreal! There's nothing wrong with that, but my projects don't require those kinds of features (and I would argue most of the things I do need can usually be learned fairly quickly).