Wouldn't that also be true of normal fireflies? It's pretty hard to track a single firefly if there's a bunch of them in the area.

Not so much, there's no competition, so it's easier to track. I used to collect freflies in a jar as a kid. Seems like a similar idea to schools of fish, herds of antelope, etc. Stay together (even if defenseless) and predators have a hard time picking an individual to hunt.

Ah, the paradox of choice (Firefly Edition).