that's a popular story, but it doesn't really seem likely if you take a closer look at the timeline: https://www.reddit.com/r/gameofthrones/comments/brj0dt/no_spoilers_timeline_of_season_7_8s_plan_and_star/
I don't personally blame D&D for cutting the series short. So much goes into making a TV show. Egos, contracts, massive workload and responsibility. This is the first show they ever worked on and it turned into one of the most massive projects in the history of television.
GRRM is sitting back being a "writing consultant" waiting for his check to roll in and refusing to help the show by releasing his books so D&D have them. When D&D started this project in 2005, they probably expected the next book to be ready by the time the show reached that point in the story.
I totally understand D&D wanting to live a life as a human being outside the production of GoT. If they had done what GRRM wanted, it would have meant devoting 20 years of their life to an adaptation without even having all the source material.
----- With all that being said, I do blame them for S8E3 being so bad. I wouldn't hold it against them for doing a bad job, but they didn't even try to give the NK a story. At least make some attempt. There were so many ways that could have tied him into the main characters that would have been satisfying.----
They made him sign a deal NOT to release the books until the show was done.