This is what I am saying as well. The 90% number applies over the full sample, not necessarily the subsample of comparable beings (i.e., a subsample of species in comparable situations where the time traveller happened to choose the ten billionth being, or so). There might be a correlation between overall longevity and making it to ten billion at all, and there might be elements in place that make a particular species more or less vulnerable to extinction by the time they get to the 10 billionth being (which are we, do you suppose?). Thus the "rules" you need to make the Carter hypothesis work categorically is that the total number of beings before extinction must be a random variable that receives no inputs from anything about the species, and no correlations with making it to 10 billion in the first place (beyond the obvious). The rules to make the snarkophilus anti-catastrophe I'm not sure I can think of, but there probably are some that could be applied in the context of this thought experiment. But I would argue that in any event, neither of those rules are plausible. As soon as you look at anything that relates to extinction potential, the odds change dramatically from the Carter "default" argument. The real question is, which way?