First you need a galaxy to from. Then you need at least one generation of stars to live and die before plants can form. You then need to give those planets enough to to form and cool down before life can show up. That's the start of the window of opportunity for life to show up. However, life on earth is ~3.8 billion years at which time the window closes and such things become visitors vs seeders of life. And this is important, all this needs to take place outside of the center of the galaxy where things are a little to energetic for life to flourish.

Not to mention you need a lot of stars to die to have enough material to create a rocky planet. A single super nova is not enough because the material is ejected over such a large an area.

Now, having said all this means is you shrink the window the distance such things can travel. Earth could have been seeded by Mars no problem, but 1/2 the distance means 1/4th the stars. Ejecta just don't move vary fast so even with a 1 billion year window you need to be really close to earth to seed it. More importantly the window between when life could survive and when life showed up less than 1/10th as long as life has existed. So, if we where seeded then we have probably had many visitors after the fact which is what this guy was saying.

PS: Of course if life is common enough both could have happened and even ridiculously unlikely events can happen.