This is a kind of strange point of agreement between the president and his critics. We're saying Bush was committed to the invade-Iraq policy, never mind the evidence, and that this attitude led him to dramatically overstate the threat in a variety of ways. Bush, in essence, concedes the point that, for him, evidence regarding the scope and imminence of the threat was besides the point.It's part of that weird disconnect in the Republican party that's going on right now- Bush and his defenders seem to be saying different things. The defenders are saying "we didn't know, nobody knew, but we have to stay the course for freedom and whatnot", while criticizing the anti-war types for defeatism and being willing to let Iraq burn. It's not the most accurate claim (people DID say it was nonsense back in 2002), but it can at least be defended.
Were Bush able to stick to this talking point, he'd be able to shore up his weaknesses and burnish his strengths. By telling an unpleasant truth and admitting to a mistake, he'd make people more confident that he's telling the truth about other things. After all, the perception that he's a liar is rooted in the belief that he simply cannot admit that he has done anything wrong- that his "resolve" has gone absolutely berserk and become dangerous. Admit to a mistake and resolve to fix it and you keep the positive reaction to the resolve and shed the negative perception of zealotry.
(Yes, I'm aware that you're never supposed to admit to a mistake in politics. There are exceptions, and this is one of them.)
That he simply can't do this when every other Republican is doing so (aside from the serious loons) only confirms the perception. He really is unable to admit to a mistake, and really is a zealot. Either that, or he's so frightened of what he's done that he won't admit to anything in fear of the consequences. Both of these are really, really bad traits in a leader, and I think people subconsciously understand that.