UE4 has been able to export to switch for a long time (Snake Pass notably released on switch, before any other console, 4 years ago)
For Godot, there are companies that do the porting (presumably they developed their own interfacing layer), these services seem to be not very expensive, and one of the Godot founders runs one of these companies IIRC.
Exactly, this porting business model is quite common in the gaming industry, and one of the reasons why most studios don't really care about 3D APIs portability.
That's not even remotely true. I know of half a dozen random 3rd party engines just off the top of my head.
i think Kingdom Rush on Switch used love2d. might be wrong.