That's correct - life being seeded on Earth explains how the Terran evolutionary branch began, but it simply shifts the explanation of how life per se started somewhere else.
Considering how old the signs of life are on Earth, and the remote chances of a life-bearing object hitting the planet, then adding on the travel time of that object plus the time needed for life to arise on its body of origin, I think the idea that life was seeded here by one impact (or a shower of impacts) is even less likely than life simply arising here to begin with.