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.

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

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

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.