The Primary Flight Controls can be operated completely by either System A OR System B hydraulics so a loss of just one system will have no effect on the basic controlability of the aircraft.
Positioning ONE of the FLT CONTROL switches to STBY RUD activates the standby system as well as shutting off access from the normal system A or B as you say.
The standby system's main purpose (IMHO) is in the "worst case" scenario of failure of both A AND B system (manual reversion) where power to the rudder and the standby yaw damper assist controlability, leading edge slats/flaps extension and thrust reversers assist performance.
With failure of only a single system (A OR B) the standby system is (somewhat) unnecessary....but nice to have anyway, in a manual reversion it shines!
Manual reversion an interesting SIM exercise - in a small way to feel how heavy the aircraft is but, more interestingly, how to best use CRM and cockpit duties to best handle an exercise that is physically demanding and tiring.
Mr Boeing says the PTU is there to provide volume of hydraulic fluid to allow operation of the LEFlaps/autoslats/slats at the "normal rate" when system B is lost; the electric pump only provides 1/6th of the volume of a mechanical pump hence the PTU to use the A system mechanical pump if required.
That's the way I read what Mr Boing says about "stuff", happy to be corrected anytime.
Cheers.