To treat someone as though they were less intelligent than they are is patronising; to treat someone as though they were less intelligent than you, when they are indeed less intelligent than you, is not. That is why we do not patronise small children when we talk at their level.
This makes no sense to me. For one thing, it is simply a mistake -- a patronizing one, in fact -- to assume that because small children have much less experience than you and much less practice with many of the things of life that they are less intelligent than you. And second, anyone who has much to do with children knows that in fact a great many adults very, very obviously patronize children. Indeed, I have difficulty seeing how someone who has ever been a child could not know this; I can think of half a dozen cases when I was patronized by adults as a child, just off the top of my head.
For another, it's not consistent with the way the word is actually used to say that treating someone "as though they were less intelligent than you, when they are indeed less intelligent than you" is therefore not patronizing: merely because you are right and more clever does not automatically mean you are not being condescending. A very handy man with a very un-handy wife can still treat his wife patronizingly when it comes to getting things done around the house -- I know more than one, in fact, who has a serious difficulty getting this point through his thick skull. It's true that when there is a clear disparity of intelligence, the difference between the one who patronizes and the one who does not will not necessarily be that the latter ignores the disparity. But that doesn't mean being right about the disparity automatically means you are not patronizing. Someone who haughtily goes out of his or her way to make people less intelligent than they are feel acutely the fact that they are less intelligent is an almost paradigmatic case of being patronizing. Everybody knows, or one would think so given how the word is actually used, that very, very intelligent people are very, very likely to be patronizing to people who are very average and ordinary: if you ask people if that's the case, they'll often say it is. And that cuts against Baggini's claim.
(The rest of Baggini's article is of some interest, although I don't have much to say about it. I have yet to see Dennett or anyone else provide any serious evidence that "belief in belief" is a very widespread problem. I have myself never met anyone who holds of any belief that it is "so important that it must not be subjected to the risks of disconfirmation or serious criticism", although claims that particular rhetorical styles or particular kinds of fora and occasions are not appropriate to criticizing certain kinds of belief are very common. Without evidence that it's actually a serious problem, it's difficult to see this whole 'belief in belief' trope as any more than wishful thinking, in which people like Dennett pretend that their critics are stupider and less reasonable than they actually are.)