"Doesn't this count as a case of group level selection?"

Yes, when it works. Divide the population into smaller groups with strictly limited breeding between groups, and that's one way that segregation distorters can be limited.

Better mechanisms might arise too, but until they do this is what you've got. There might be some other advantages to a population divided into small groups with limited interbreeding, too.

And once you have a population divided that way, it leaves the possibility for other group selection. However, rats are mammals, and mammals are a small minority group that are unimportant in the bigger scheme of things. How often are species divided up like that?