HD, keep in mind that C++ DLL compatibility rules mean that any code that links to UE4 must be recompilable from source, else it will be incompatible with code changes made by any developer who alters base classes like AActor, etc. If a developer were dependent on a whose UE4 binding couldn’t be recompiled, that would fundamentally break the entire UE4 source code development path for them.
So, the issue at stake isn’t whether or not a given ships with source, but whether that breaks the entire UE4 source development path for developers who use it.
Thus the expectation of requiring source for the portions of a plug-in that link to UE4 while any engine-independent code may remain closed source.