If they’re following at that velocity, then we’re already dead, and “they” are a relativistic bombardment we won’t even see coming.

Depends on how far away the signal came from. 10^9 light years away, and your .98c attackers won't be here for 20 million years. Even 10000 light years away, you have 200 years to play with.

Though it's likely pretty hard to get a signal 10000 light years, especially if you don't know the direction to send it.

Then the promise of safety is all the more tempting, isn't it?

Maybe, but if they’re sending us EM signals then we know we’re doomed. Physics as we know it, with c as a limit, strongly implies that it is simply impossible to defend against a relativistic strike.