This depends on how fast they’re spreading physically. If spread rate is close to c, I don’t think that’s the case, I think it’s more likely that our first contact will come from a civ that hasn’t received contact from any other civs yet (and SETI attacks would rarely land, most civs who hear them would either be too primitive or too developed to be vulnerable to them, before their senders arrive physically.).
Additionally, I don’t think a viral SETI attack would be less destructive than what’s being described.
There is a greater chance of observing self-replicating SETI messages than those that destroy planets
This depends on how fast they’re spreading physically. If spread rate is close to c, I don’t think that’s the case, I think it’s more likely that our first contact will come from a civ that hasn’t received contact from any other civs yet (and SETI attacks would rarely land, most civs who hear them would either be too primitive or too developed to be vulnerable to them, before their senders arrive physically.).
Additionally, I don’t think a viral SETI attack would be less destructive than what’s being described.
Yes, SETI attacks works only if speed of civ travel is like 0.5c. In that case it covers 8 times more volume than physical travel.
And yes, it will be also destructive, but in different manner: not bombs, but AIs and self-replicating nanobots will appear.