You may have missed the part that he is CANADIAN, not a USA Iranian immigrant. It was his CANADIAN passports that were in question; and note that one is in his ‘real’ name; the other in his alias’ name.
Note that as a non-immigrant, temporary visitor to the USA, he, in general, can not purchase firearms.
The statement he made about ‘going hunting’ was an attempt to cover his tail by asserting a permissible exception, PROVIDED he has the proper hunting license...and he didn't.
The crime leading to confiscation is most likely illegal purchase/possession by a nonresident alien, but the 'false staements' charge was enough to hold him while the rest gets investigated.
Possession of firearms by non-resident visitors is legal, even if they are residents of Canada and an Iranian national. That does not mean he entered the country illegally. Whether he purchased them here or if they belong to someone else has yet to be determined.