The proposed genetic bottleneck around the time of the eruption was long ago - when the human population may have been very small anyway. Today, we have six billion humans. There are better defenses against such things - in terms of stocked underground bunkers. So: a modern volcanic eruption would have to be vastly more destructive to kill all humans. The probabilities involved are miniscule, and shrink with every passing day. It is only because of a "Pascal's wager"-style argument that people can be made to consider such risks.