When I am inclined to call someone childish, it's because I want to express an opinion about their maturity, not their intelligence. Smart people can be immature and mature people can be pretty dim.

Some people really seem to be able to self-deceive without obviously weakening the belief they're deceiving themselves about. It's not a skill I have, but I shouldn't assume that no one has it. I don't think it's obvious at all that these people are necessarily dumb.