I'm not the grandparent poster, but I think that "even if they moved at a fraction of speed of light" refers to the responding civilisation's space vessels, not their return signal.

Let's say you receive an EM signal from a sentient specie.

If you pinpoint its origin to light years away, you know they already had a lot of time to progress past this technology. A good response if you plan on eliminating them would not be a fleet: you'd better setup and direct a huge gamma-ray burst in their general direction.

So the destructive response could come at the speed of light with a little delay at first.