> Yeah, but during the impact we need to be somewhere else.
Bunkers should be fine, the deep sea may be fine, moon will pretty much certainly be fine, high orbits should be fine as well. If you can manage to be on the other side of the planet, surviving the impact should be quite manageable if you have the kind of resources that can build a deep, shock-resistant bunker, and you should be able to do that much, much easier than build Mars colonies that are independent enough for this, and large enough to not just save a hundred people.
After the initial effects wear off, you're probably facing conditions topside that are absolutely brutal, but still way less so than Mars with its toxic soil and dust, thin, useless atmosphere, supercold temperatures, very low gravity, scarcity of easily-accessible nitrogen (let alone easily accessible metals), high radiation environment, ...
Basically, if you can set up a naval nuclear reactor and some lighting and greenhouses, you should be able to get food production going in spite of any realistic asteroid winter, the air will still be breathable, the soils will still work for agriculture, ... and you can make use of all that survives of our artifacts and civilization, which should be a lot. Mining former cities for resources could be a big jump-start.
Yeah, but during the impact we need to be somewhere else.
Making Mars habitable for a year might be better than making Earth habitable during an impact.