Question about default godess tower event

Assuming you have a M!Byleth leading the Blue Lion House on classic mode, what happens with the godess tower event on the night of the ball when every available student is killed? Does it default to Hilda or Flayn because they don't die upon defeat? (Or maybe they do, I'm unsure)

I think to remember a special gatekeeper conversation but that could've just been a fever dream I had back in 2019.

Any answer is appreciated, but I will test this myself within 3 days if necessary.