Indeed, and as a biological weapon it is quite practical: it kills almost everything non-plant life forms, and if there are survivors the hybrids will take over. But such hybrids creatures cannot be multiplied by reproductive methods and will probably die eventually. So you can clean entire planets with that, destroy civilization, etc.
The only thing that is needed is for the Juggernauts to have weapons to avoid being knocked out by enemy fire. Although considering that the attack must have occurred 2000 years ago, it makes sense that they wouldn't expect humans to be able to defend themselves.
There was an entire fleet in the silos of Planet 4. If they had attacked millennia ago, the humans would already be dead and
Prometheus (nor the
Alien quadrology) would never have happened, in-universe speaking
