We chose not to tell our kids about Santa as well. I'm glad to hear that it worked out for you! Our oldest is 3, and she does know about Santa, but she thinks of him as a pretend story, just like mermaids and dragons. Fun to talk about or make up stories about, but not real. We also try to limit how much we talk about him around Christmas, because we don't want Santa to be the focus of Christmas. I remember being somewhat traumatized when I still believed in Santa as a kid, and a friend taunted me, "You don't *still* believe in Santa, do you?" I was a gullible kid!
I'm wondering, with some of the PPs, how it comes up? I get what you're saying, that everybody knows, so it comes up. But is he bringing it up from time to time just to tell you how wrong you were? Like without it even being the conversation, just spontaneously? Or is it coming up as a lead-off from other conversations that maybe could be avoided with him? And can you give us some samples of what he says?
I'd probably go with what the other PPs basically said, to tell him that his questioning of your parenting choices is unnecessary, that DD obviously turned out fine, etc. Unless you particularly like him and *want* to (and it doesn't sound like you do), I wouldn't go into your reasoning. It probably won't change his mind if he's a know-it-all as you say.