As Dubya himself said, "we misunderestimate" him. Dubya achieved a level of stupidity which will likely be unmatched by any other president.
While I don't think that Bush was particularly interested in policy or the application of policy, I do think that he was quite intelligent. Particularly when it comes to playing to a crowd (Karl Rove got too much of the credit there). And that quote you've just attributed to him is one of the best examples of how good he was at it. The vast majority of people would understand what he intended to say. Those that would criticise him for it would come across as aloof and intellectual snobs to those who weren't offended - his supporters and those who did not have strong feelings about him. Those who weren't offended would view those who were in a more negative light. This negative feeling would expand to most people who opposed Bush, including people who didn't see it as something worth criticising him for (guilt by association).
Yes, you (and many others and me) did "misunderestimate" him. I was strongly opposed to many of his policies but people making the easy criticism of him for misusing the English language detracted from [legitimate] policy criticisms against him and tarnished anyone opposed to his policies.