Certainly within the realm of plausibility, at least. Any race capable of space flight would be able to simply nudge an asteroid or three towards the planet. Heck, we could do it with existing technology, if we had the inclination and a death wish…
that would be the best Mythbusters episode ever! “Well, Jamie. Looks like this myth is confirmed. With existing technology, you CAN nudge an asteroid toward earth.”
“Well, Adam. That certainly seems to be the case. So not only did we confirm a myth. I think this is probably going to be our best explosion ever!”