While totally believable it still totally sucks. If your job was to write this book, wouldn't you lock yourself in your writing space and do only that.
That's not his process. That's never been his process. And writing at his level isn't something you can force yourself to accomplish.
I feel kinda bad for the guy. He spends his entire life doddering along, writing his books as he wants to write them. Totally content. Then all of a sudden HBO decides to adapt a story and now the job he loved causes him nothing but stress. I'm sure the fortune and glory take away some of that sting, but it's not hard to wonder if, given the choice, he might not go back to obscurity.
My only worry is that he doesn't finish the books before he meets the same fate as the Starks. He's no spring chicken. I'm sure he has enough of the last two books written that an apprentice author could complete them as happened with the Wheel of Time series, but those last books weren't near the quality of the rest of them.