I'm probably just not remembering it correctly but I know of no way to apply updates to a locked iPhone. Even when unlocked if you plug it into a new computer the phone will ask if you trust the new computer. Is there some method of updating the phone when it's locked I'm unaware of?
I think it's some kind of device recovery mode designed to recover from "bricked device" situations. It's not the same method that you use when updating via iTunes.
