You are right, but why rely on a single method of protection? Vaccinations are good, but not perfect.
Think of it as using a condom despite being on birth control pills: you are not just unlikely, but really really unlikely to get pregnant and it also protects from stds.
I have never though about it that way, it is an interesting thought.
I believe, that some people put on that Covid denialism because they are unable to face the truth out of fear. And every mask is a reminder of that fear. I don’t have proof, but I know people who work that way with a lot of things, from climate to covid to Russia.