"It was not an asteroid or comet, because it would have killed everything"

This quote by Robert T. Bakker is referring to an event that occurred in the past. He is recommending that whatever took place was not caused by an asteroid or comet, since if it had been, it would have had disastrous effects. This suggests that whatever occurred was not as devastating as an asteroid or comet would have been, and that whatever it was, it did not cause the destruction of whatever. This could mean that whatever happened was not as effective or damaging as an asteroid or comet, or that it was effective however had a more localized effect. It might also imply that whatever happened was not an asteroid or comet at all, however something else completely. Whatever the case, it is clear that whatever occurred was not as destructive as an asteroid or comet would have been.