Not true: any old asteroid strike launches millions of microbe-bearing rocks into interstellar trajectories. Any that do reach another planet might take million

Not true: any old asteroid strike launches millions of microbe-bearing rocks into interstellar trajectories. Any that do reach another planet might take millions of years to get there, and be vanishingly unlikely to pick one that microbes can survive on. But it is wholly possible, and wholly possible that we got infected in exactly that way.