Two things.
It was corroborated to the level of a Presidential briefing. Do you know how much intelligence gets there?
And second, Bush has said there was no warning, and Rice has said there was no warning. They both lied. Lied long, lied hard, lied often.
And they are lying about a lot more. It's only by accident the public knows about this memo - obviously there is more which won't be released. What are the odds of US finding out about the only one?
The True Believers will twist and turn this stuff, and they can't deny either of those facts. Which, by the way, are damning unless one is, after all, a True Believer.
Arf