That sounds like a very reasonable explanation to me. That being said, I fully expect a new chipset to be launched with 10nm, even if this new socket and Z390 could technically work.Agree completely.
My favorite theory is that this new version of the socket was intended (and is still intended) to deal with the demands of the 10nm CPUs, when and if they get here.
But I won't say the pins aren't needed for the i9-9900K, there's just no proof of that.
Its just what Intel does.