From the FB thread:
Nathan Bouscal: Note that I haven't heard significant disagreement about a colony being useless-ish against AI x-risk. The argument is that it helps with (almost) every other x-risk.
Robert Wiblin: Even then the disagreement isn't that a Mars colony couldn't help, it's that you can get something similarly valuable on Earth for a fraction of the price and difficulty.
Paul Crowley: The proper disagreement to measure is something like "A permanent, self-sustaining off-Earth colony would be a much more effective mitigation of x-risk than even an equally well funded system of disaster shelters on Earth."