...but about the question of the original poster. I am curious too.
When both engine bleeds closed, why can't we use the APU?
It can't be because of contaminants, because the checklist doesn't end, like framer pointed out. Also the contaminated air would already have entered the cabin, and it would probably only be a few seconds to blow the ducts clear.
Hence, it must be because we don't want bleedair ducts pressurized? But if the engine bleeds are closed, why would it matter? Also the starter valves are closed so bleed air would enter the engine area through this way.
Or maybe it is just written that way to avoid any mess up, that would complicate the situation further? I am leaning towards that option.