Yeah but they would need interstellar-type of timescale to reach us anyway even if they moved at a fraction of speed of light, so the risk is close to nil. We are way, way more likely to be hit by an asteroid or to destroy our planet than to be destroyed by a very remote alien civilization.
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.
Yeah but they would need interstellar-type of timescale to reach us anyway even if they moved at a fraction of speed of light, so the risk is close to nil. We are way, way more likely to be hit by an asteroid or to destroy our planet than to be destroyed by a very remote alien civilization.