I think it's just speculation on the fact that it's only been three years since GoT launched and games take much longer to make. There's no way GoT2 (if real) is in any showable state considering it's two years away at best.It is peculiar how Grubb knows GoT2 won't be at the Showcase. Usually it's things he knows will be there. I wonder how this info came to him.
Pair that with Sony touting their live service initiative, Spider Man needing a blowout, and third party deals, it just doesn't make sense to show off a GoT2 that is nowhere close to done.