please explain how a meteor would guarantee the extinction of humankind, and if so, over what time frame this would occur. I think you're overestimating the severity of a large meteoric impact on a geologic time scale. Mass extinctions occur over thousands of years. Homo sapiens' ancestors survived every one of them so far, by definition.
At a sufficient magnitude, a meteor might well liquefy the Earth's crust, or cause seismic shocks sufficient to damage even the deepest bunkers.
That said, such an event might well cause problems on a (far more generally vulnerable) Mars colony as well (e.g., ejecta subsequently impacting Mars).
And the odds of such an event are quite low. Most killer asteroids would disrupt the biosphere, and quite considerably, but not significantly rearrange the Earth's structure itself. The one time that's happened which we are reasonably confident of is the hypothesized Theia impactor, roughly the size of, erm, Mars, which is thought to have impacted the proto-Earth as the latter was forming 4.53 billion years ago, and forming the Moon as a consequence.
Other cosmic events such as a gamma ray burst or rogue star disrupting solar system orbits would have impacts affecting Mars as well, and even a well-established self-sufficient Mars colony would not be a sufficient mitigation.
(I'm generally not in the "Mars as a lifeboat from Earth" camp.)
