Pretty sure Lewis was upset because he couldn't prove his Christian faith as rational, so had to invent a lot of ways to discredit modern thinkers.
I'd go as far as saying that "chronological snobbery" is pretty much non-existent. Look at all the ways people idealize the past, especially when they talk about the 'good old days' or how things are much worse today.
Of course, the other extreme of idealizing the past exists as well, and is just as bad an approach. However, one does frequently hear people talking about people in the past in a very derogatory way ("I can't believe people used to believe xyz..."). While we do know a lot more nowadays than we used to, many things that we claim an "enlightened" perspective on are not actually facts, but mere interpretations of the world based on our cultural context. In effect, chronological snobbery is nothing but cultural snobbery projected back in time. (If you've ever lived in another culture for any significant period of time, as I have, you will realize just how differently two completely rational people can view the same event through different cultural lenses.)
As to Lewis: as far as I am aware, he actually coined the term "chronological snobbery" (or at least the concept) before he became a Christian. He did, after all, teach old/medieval English Literature at Oxford and Cambridge, so the past is something he dealt with and thought about a lot. And if you think he couldn't talk about his faith rationally, try reading some of his books (especially something like "Miracles")...
> If you've ever lived in another culture for any significant period of time, as I have, you will realize just how differently two completely rational people can view the same event through different cultural lenses.
The best course on American History that I ever took was taught by an man who had grown up in South America (he was teaching in the US). I first learned a number of interesting ways of thinking about the Americas through because of cultural emphasis that he brought from another part of the world.
I'd go as far as saying that "chronological snobbery" is pretty much non-existent. Look at all the ways people idealize the past, especially when they talk about the 'good old days' or how things are much worse today.