I'm an amateur here, so take everything with a pinch of salt.
CE is not one large drill like the one they used for the channel tunnel. It is a tradesman's bag full of tools like a hammer, a hand drill, a micrometer, a chisel, several screwdrivers and spanners, etc
So if CE 3.7 had all these tools and CiG changed the micrometer and the chisel but left the rest... And LY took the 3.7 toolbag and have left it more or less alone but added some Whitworth sockets and a pair of snipe nosed pliers...
The upshot is that most of the toolbag is still the same, with a couple of additions from both sides that they need to check for interoperability?
I think it is getting fractious in here because everyone (I'll admit it was my thought), has been getting hung up on the old "50% modified" quote from Brian C?
Something largely based on CE 3.7 being flipped to work with something else largely based on CE 3.7 is nowhere near the pain of going from CE 3.7 to Unreal 4?
Just trying to calm the waters a little, although I do reserve the right to be wrong!