Why not add more steps? The aliens¹ brought microbial life on an interstellar ship and seeded it. A lot better chance than riding out for a few million years on

Why not add more steps? The aliens¹ brought microbial life on an interstellar ship and seeded it. A lot better chance than riding out for a few million years on a piece of rock! See, it's approximately on the same trailing end of unlikely as panspermia, but I fancy this one more. ¹Let's call them Engineers