It means that you can switch the kernel beneath the userspace and hardly notice. So the Linux in GNU/Linux can be replaced, giving you GNU/NT, I guess.
Debian actually tried this a while ago with FreeBSD, but I think it was discontinued.
It does not mean anything. It's a minor variation on a commonly copied piece of text that has been doing the rounds for years, by people placing it in discussions for fun. It's not actually a genuine argument.
