Dave,
Check just over a third of the way down the article. You'll find this line:
It may be little known to those who neither own nor care about firearms. But for folks with even a modest amount of knowledge of gun laws, it is well-known that exemptions for antique/replica/muzzle-loading arms are commonplace. Certainly the police should be aware of this exemption when they go to visit a guy with an arm that reportedly violates local/state law.The loophole allows license-free ownership of "antique firearms" - defined as rifles that require the bullet and gunpowder to be loaded separately.
Yes, his case does "depend" on that exemption. Describe it how you will, it appears to me to be an open-and-shut case if the exemption is as stated in the article. So, it is not "clinging", or "dependant upon a given interpretation", it is clear (again, IF the exemption is as stated in the article).