That's provably wrong. In the first place, both his definitions of "smart" and "stupid" are wrong according to the dictionary. You can't just redefine words so that they suit your argument. If that's an acceptable approach, I can just as easily say all living humans are stupid by defining "stupid" as "not Einstein".

Secondly, even if we allow that his private definitions be used for the purposes of this argument, he's still wrong. People do exhibit consistent behavioral tendencies, after all. Some people consistently think things through, while others consistently jump to conclusions. So I think there is pretty good justification in applying the heuristic of thinking a person is stupid if he consistently jumps to conclusions.

I hate to say this, because I have much respect for Derek Sivers, but this post was pretty stupid.