So I have a structure that has public docking in highsec but no other permission allowed. I just got a series of notifications that Blood Raiders, yes the NPC Pirate corporation, were attacking my structure. No one was able to sit in the seat so it isn’t possible that someone simply shot at the rats with it. They did no actual damage either.
Five frigates should be able to meet the minimum DPS (like 400 DPS for a medium structure), and as long as they keep applying damage, the reinforcement window should stay open until it gets reinforced, even if that takes hours.
I would say either the same mechanics don’t apply to NPCs, or they are fickle and wander off after a time before they finish reinforcing the structure.