I was surprised you'd give the human race so low a chance as a coin-flip.

I think the reason Roko puts it so low - and at the probability one gives when one doesn't know either way - is because there are so many current existential risks like nuclear warfare, pandemics, asteroids, supervolcanoes, and there are multiple existential risks which could become a concern between now and when cryo-resurrection is feasible (grey goo-level nanotech is a plausible prerequisite for resurrection, and equally plausible candidate for destroying humanity, to say nothing of AI risks).