Having just come from a university environment, I can say without question that this sort of thing happens to young college-kids ALL THE TIME. Remember, unless a person is being held to a professional code of conduct, they can pretty-much say WHATEVER they want to. In the case of young and inexperienced college-kids, often times landlords (school officials, LEOs, auto mechanics...) will say something that is so false and inappropriate that, if an attorney said it, they could get disbarred!
In this case, I do not believe that the landlord has any right to force the kid to get rid of the weapons. (I wouldn't rule out the possibility, though. Those landlord-tenant contracts can be awfully convoluted, and the "dangerous materials" clause might prove bothersome...) But, moreover, it sounds like this is a landlord (slumlord) who rents sub-standard housing to college-kids, refuses to obey the laws of building requirements, and then huffs-and-puffs to any college-kid who has the audacity to tell him to stop.
This guy (and the millions of others like him) makes a living based on two principles: First, the rent FAR outweighs the maintenance. If he has to fix it, he won't make any money. So, he will do WHATEVER it takes to avoid fixing the house. Second, any college-kid naive enough to rent that house very likely has no money for a legal battle, and has NO IDEA what the landlord can and can't get away with! So, if there's any trouble, he just puffs up his chest and threatens to evict the kid. The kid will wet himself, and then promptly shut-up and continue living in squalor.
I've seen this happen to so many of my friends. And, I've intervened (and scared the hell out of some local slumlords) where I could. It's sad, really.