You don't think he could've initially thought we'd be spending two or more seasons with each book? Consider the context in the post for what you quoted:
Having said all that, I know what the next question will be, because hundreds of you have already asked it of me. Will the show 'spoil' the novels?
Maybe. Yes and no. Look, I never thought the series could possibly catch up with the books, but it has. The show moved faster than I anticipated and I moved more slowly.
Of course if he recently thought he still had a chance of staying ahead of the series, that's a special kind of crazy. But my interpretation of that line in his post was more of "look, when I agreed to do the shows, it didn't occur to me we'd have this problem of the series surpassing the books". And I buy that.
Let's say the first season was obviously going to move quickly and cover the whole first book, but if he thought the rest of the books could each take two seasons or more on screen, A Feast For Crows would've taken us through at least seven seasons (2017), giving him until 2020 to finish The Winds of Winter and 2022 for A Dream of Spring.
In that scenario he would still be fine with his current pace. It might've always been a naive view of the TV schedule, but it's not crazy to think that when the show was starting, it never occurred to him even with his slowing pace of writing that the show would catch up. And that's the concern I think he's addressing in the quoted passage.