Yeah, the alien has never needed to breathe, neither in the movies nor expanded media. It survives in vacuums, underwater, different atmospheres (I mean, it'd not be much of a perfect organism if it popped out of a host on a different world and just instantly died because it was oxygen dependent).. the one and only time we've seen anything that resembles breathing is the sound the Queen makes... but without more intel on xeno biology we don't actually know what it's doing... or what it is taking in /putting out (if anything). Also that same queen survived perfectly fine holding onto dropship landing gear on its way back to the Sulaco in orbit (so not just the seconds we've seen them blown out of an airlock).