Hard to say. You claim they are incapable of understanding, which is why they can't be fluent. however, really, the whole argument boils down to whether they are capable of understanding. You just state that as if it's established fact, and I believe that's an open question at this point.

So whether it is circular depends on why you think they are incapable of understanding. If it's like the other poster, and it's because that's a human(ish) only trait, and they aren't human...then yes.