Considering that the armory in question appears to be under some construction, the alarm may not have even been on.

Most armories I’ve been in could be cracked by my Boy Scout Troop.
A good set of bolt cutters and a torch, and you’ve cracked the locking rack (locker).
Most of these places are more dependent upon the outer doors and occupancy than physical strength (safe).

Remember, the feds don’t have to comply with the same laws a regular person would have to, regarding Class3 weapons.
Ain’t exemptions great.

Again as to what they got, its a guard armory, not a military base, the heavy guns and equipment isn’t stored there. Just a token number in case of emergencies.

Still am not convinced this is any more than some opportunistic yokel rather than a year, its just too easy to gather stuff through normal channels or foreign suppliers.