The asteroid that killed the dinosaurs 65 million years ago (10 km in diameter, mass greater than 1 trillion tons) must have ejected billions of tons of life-bearing meteorites into space. …
So many ifs and buts in those calculations.
The bacteria would have had to survive 3 major events:
1/ heat near vaporisation being ejected from earth;
2/ near a million years without food in the vacuum of space with all the radiation issues leaving and re-entering solar systems;
3/ heat of re-entry into the destination.
In practice, we find that we can sterilise food effectively by temperatures as low as 115 C (pressure cooker).
So while their calculations with respect to rocks reaching far away places may be correct, it seems extremely unlikely that any of those rocks would contain life forms capable of self replication in their destination environment.
Thus the probability of life travelling interstellar spaces seems very low indeed – much less than 0.001 (so rounded to 2 significant digits, much closer to zero than to 1).