Capn Bloggs,
The standby power switch on the diagram is located in the flight deck, it's not the same switch that's operated from the exterior airstair control. But i'd say you are right you can bypass the battery switch via the "alternate thingee"
Had a quick look in the 737 cockpit companion (or if it was the MRG) together with my colleague today. It said something different to the FCOM, if we read it correctly:
a) The airstair standby-mode is powered by the Sw. Hot Bat Bus, and
b) Holding the exterior airstair switch to standby will power the Sw. Hot Bat Bus until released, even if Bat switch is off.
That would explain my question, but the "official" source, i.e. the FCOM says something else.