The full quote:
“I will simply say, I made every effort to include George in the adaptation process. I really did. Over years and years. And we really enjoyed a mutually fruitful, I thought, really strong collaboration for a long time. But at some point, as we got deeper down the road, he just became unwilling to acknowledge the practical issues at hand in a reasonable way. And I think as a showrunner, I have to keep my practical producer hat on and my creative writer, lover-of-the-material hat on at the same time.”
Quite frankly after what Dan and Dave to ruin Game of Thrones reputation, this honestly isn't surprising whatsoever.
They cut out a character to save money and air time, and George thinks that will make the plot fall apart later.
Regardless of who's right, he's probably not going to trust a showrunner who's trying to rush things given past trauma.