Thats what common interface exacly is for, how do you think UE4 works on some many platforms? 
Because different APIs are wrapped with common one down to level of C++ core liberies (thats why you don’t use standard C++ even if you can but only UE4 APIs). UE4 unifies all online services (Steam, Xbox Live, PSN etc) under common interface (onlinesubsystem) as well as render liberies (DirectX, OpenGL etc in to RHI), and i think VRs to in 4.8 (not exacly sure).
What is common in for AR technology which UE4 game code will need, first camera support which UE4 missing (which is need to be supported by platfrom interface), the camera positioning and tracking state (if glyth i lost or not). Epic can create common interface for such things so they can be used in any platfrom and any technology, it will just need module or plugin that will operate those APIs, which will be brige between UE4 common API and specific technology plugin.
I think it will be lot nicer then making those companies implement AR in different incompatible ways, it would be a mess