If you engage people robbing the building. Aka with a gun out and die you can't be pked (or just gunned down unarmed) but if you're caught off guard they can stick you up and mug you, which is where fear rp applies. So if you're being mugged while they rob and you break fear rp and are killed it's a pk. It just basically becomes a mug.
The initial rule is there to clarify that if you are gunned down (RDM'ed) or killed while attempting to defend the building from an attacker/lockpicker its not an auto pk for retaliating.