I agree with your skepticism of the author's conclusion. For example, we only know that the location/timing of the asteroid impact was so devastating because it

I agree with your skepticism of the author's conclusion. For example, we only know that the location/timing of the asteroid impact was so devastating because it did in fact hit there and so we were able to study it and find the "smoking gun", in other words, it wasn't obvious. But how can you assume that if it hit somewhere else at some other time that we wouldn't have found some hidden life-ending mechanism in that event as well?