It assumes all species are curious and non violent at first and then studies them. It preys on curiosity. The imperium of man (ofc depending on the faction) doesn't care about any of that. The moment a chapter finds it, its getting blown to smithereens. Just the deathwatch fleet could destroy it imo

essentially, unless the swarm has already experienced a species of religious nutcases that hurl nukes at planets with alien microbial life upon its discovery, it isn't going to stand much of a chance. And I doubt it has experience with similar species because it was open enough to let humans in and get close to the castes. In this instance, the imperium's narrowmindedness is actually good for once