You are $21T in debt. That ship has sailed.
It goes up faster than you can read the amount.
Your looking for DOJ's Appellee brief, which is behind PACER. I've downloaded it and dropped it on a google drive. Specifically, the bottom of page 40 through 42 or 43.
You would have to go to the court records of all the filings in the case and read them for yourself. The filing may even be published online, but that's not a guarantee.
If you really want to swing for the fences, you can interpret that as the ATF saying "The Circuit ruled in our favor but they were wrong to do so." The New Civil Liberties Alliance seems to be really running with this, as if the ATF were completely throwing in the towel. That's not what's happening. The ATF is, at best, admitting to the possibility that the Circuit ruling in their favor was wrong on this one point. Only.
Thus, the ATF argues that this particular point, while having contestable merit, isn't determinative. They are arguing that it isn't really on-topic since their Rule isn't based on "gap-filling" (making up stuff to fill in the gaps left by incomplete statutes) but, rather, on a newly-correct understanding of the language of the NFA.
