For starters, he is impressed that Bush can get through the following simple sentence without stumbling: "To lay out my juvenile-justice plan in a minute and a half is a hard task, but I will try to do so." The bar seems lowered here, but there are other examples. Fallows points to Bush's ability to stay "on message," which is a rhetorically savvy strategy, but hardly the basis for "winning." Again, we're back to definitions, which are not easily dismissed.
His non-stumbling is only mentioned because he has a reputation for stumbling. As for how you define "winning" -- well that is the whole point. You obviously define winning in such a way as to align with your notion of IQ. And if you were running for president against George Bush, you would probably lose. But if one measures IQ by achievement, then, no matter which way you slice it, Bush is one of the smartest men on Earth, since he has achieved a tremendous amount. Ultimately what matters is accomplishment -- not potential accomplishment. You seem to be thinking of IQ in terms of potential. I'm using hindsight and pointing out actual achievement.
Fallows says that neither Bush NOR Kerry has lost a debate in what he calls "the only way that matters: a serious post-debate decline in the polls or an electoral defeat." That's a pretty shaky way to define winning, in my view; it assumes that people watch the debate and then change their mind if the candidate does poorly. (The criteria for how poorly a debater does are subjective and usually partisan when you're talking about presidential politics.) Are post-debate poll results the "dog eat dog" world you're talking about? Fallows makes it clear that Bush "wins" in debates primarily because his handlers set the expectations so low. As long as his opponent doesn't force him into a major gaffe, Bush "wins." (I'm not the only one using the quotation marks; Fallows does it, too.) He quotes Mary Beth Rogers to this effect throughout the article.
You imply that winning in "the only way that matters" is a shaky definition of winning. I think it is a pretty good one. Would you be more impressed with Bush if he conformed to your idea about what it means to win a debate and then went on to lose the election
Frankly, Bush is savvy enough to know that presidential debates are not about the nuanced exchange of ideas. Anyone who thinks that they are is not that smart.
Likeability is a key factor. Bush seems more likeable, so most viewers overlook the substance of W's remarks (or lack thereof) and base their judgments on who they'd like to have a beer with.
Sure. And Bush know this. And he exploits it. Gore, on the other hand, seems to have had no idea what people were looking for. He kept huffing and sniffing and then kept changing his image. It turned people off. He didn't know how to be likeable, but Bush did. But you want to define intelligence in this context as the ability to recall obscure details of policy -- which is simply not what the debates are about.