I don't think there's any truth in the claim that GoT was shortened to get D&D to Star Wars faster. This last season of Thrones may have had less episodes, but the production period was a lot longer than usual. The season before that, again, less episodes but a longer-than-usual production. We've known both these seasons would be shorter since mid-2016, but the announcement about the Star Wars series only happened in early 2018, when season 8 was halfway through filming.
So unless the Benioff/Weiss Star Wars series has been in pre-prod for years before its announcement, I'm pretty sure there's zero substance behind the allegation.
My opinion, they didn't have enough pre-written material, and the writers weren't up to devising Martinian plotting. Quite why they didn't get Martin himself in the writers' room, I don't know, but IMO that omission was a big mistake.