It’s a false equivalency on the Ratman’s part. There exist rats. There exist men. He’s a cross somewhere between, where the man is just a man. I mean, provided there isn’t some kind of miniature, unintelligent human analog in this world.

And ofc humans are called “man man”. That’s what homo sapien roughly translates to.

We are all men. He is a “rat”-man, I am a “hu”-man.

So what does he want to be called? Just a rat? But - there are actual rats, which are already called rats. Unless those don’t exist in this universe, “ratman” seems like a pretty reasonable term. He has both rat and humanoid features.