You're allowed to fly with a gun (in a checked bag), and it doesn't make sense for TSA to enforce a law in America that isn't a law in America. It makes more sense to let them get to Australia, where they've then broken the law, and arrest them there when they get off the plane.
Isn't this from the video where some lawmaker or something was like "These guns shouldn't exist so I'm gonna destroy mine" but they actually just made an illegal SBR?
Could also be a misclick with the markup tool after taking a screenshot