It's an interesting distinction, or if there should be one, between libraries-that-provide-value and libraries-that-must-be-used-to-access-a-platform.
If I create a platform, and an SDK that targets that platform, and require everyone to pay for the the SDK to use my platform...
... is that the same as if I create a set of libraries (nee game engine) that avoids you from having to recreate that work yourself?
I'd hazard that the two are different, because Epic isn't really gating access to an "Unreal Device" by withholding access to their engine.
A developer is free to use a competitor's engine and distribute the same places. (Afaik, Epic's store doesn't have any "must be built on Unreal" qualifier, right?)