Typically saying "illegal firearm" means it is illegal for anyone to own the firearm in question regardless of the person's status or how they got it. If a person is barred by law from possessing a firearm then it is possible for them to have a "legal firearm" in their possession; for example, if someone not barred by law owned the exact same firearm then there would be no issue.

But I read the post in the manner that I'm guessing was intended, so I personally don't see it as a big deal.