Grant knows that killing most of humanity wouldn't help things, because you would know that you did it and therefore mucked with the probabilities. He is saying that if you did such a heinous act, and the survivors had no knowledge of it other than where they fell in the order of humans born, they would in fact conclude that humanity's "clock" has been extended. It's a little like playing poker. If your opponent has four of a kind, he/she is pretty sure they'll win and will bet in that expectation. But if you're sitting on a full house, you say, "bring it on". It's not until they see how you are betting that they begin to get an uncomfortable sensation.... The issue is, probability is not an absolute thing, except in quantum mechanics. Everywhere else, it is a matter of information. So the question really is, what information do we possess that can give us a better estimate of our "chances" than the Carter hypothesis, which is what I meant by, in the complete lack of all information, the odds are always 50/50, but that's a pretty meaningless way to inform your decisions! This is related to Fram's initial point that there may be better ways of estimating our longevity than using the "default" thinking of the Carter hypothesis.