That is not hard vacuum like in space. I work with systems that generate so called Ultra High Vacuum, which starts at about 10^-8 mbar, but is more typically 10^-10 mbar - 10^-12 mbar. That corresponds to the pressure at the altitude of ISS (about 400 km).

Atmospheric pressure is 1000 mbar and the pressure in that video is probably 0.1 or 0.01 mbar. Lots of oxygen left in that "vacuum".

When exposed to hard vacuum, the free oxygen will leak out of the cartridge and KNO3 (saltpeter) may become decomposed, too. So, fresh ammo that's been exposed to atmosphere will fire, but if you store your ammo in hard vacuum, it's likely to fail.