The AFM (as I mentioned earlier) specifically refers to the start-up test of the flight guidance computer. It might be possible that this (powered from the secondary feed bus) needs power and that bus is now not powered (it would be powered with either APU gen or Ext power online).
Another possibility is that by having the bus tie closed (as you would have for normal starting) without APU or Ext DC power, simulates double TRU failure with both DC gennies operating - and that would cause main-secondary ties to close. Now you have one 300A generator powering ALL buses AND still recharging a battery that just took a 1200A hit.
Sorry, I can just give you my best guess. We either start both on GPU/APU or follow the full battery start procedure.