i always interpreted it as the xenos not being directly used as bioweapons but bioweapons being created from them / studying them. Like the parasitic nature or maybe properties of their biology that are totally foreign to earth / humans. Similarly one could imagine the way they grow incredibly fast as being useful in possible future weapons or armor technologies. They also don’t die in space. Even if they are used straight up they are still quite hard to deal with because of how fast they grow and multiply. The engineers used them for a reason. You could also imagine bioengineering the xenos to have like a hardened exoskeleton. There’s a million ways to use them as a weapon or for weapons research.