There are two ways to figure a probability: Calculate it or estimate it. I think the "once every 100 million years" assessment falls into the latter category -- though we do have a much better idea what's out there nowadays than even 20 years ago and can do a better job calculating these probabilities. You're definitely correct, though, I was only referring to the "unconditional" probability of a 6+ mile bolide clobbering us.
Can one (or some) coming from God only know where (no disrespect meant), sling around some star(s), planet(s), moon(s) and end up traveling at google speeds?
Thus arriving at their ultimate destination sooner than anyone might expect?