I don't think that follows. Whether people are spread out or clustered won't impact whether they engage in catrastrophic activities as long as the same number of people.

For many of the one's you've mentioned they do stop fairly locally as long as there's a big separation. Diseases jumping from continent to continent is much easier than jumping from planet to planet. Similarly, a grey goo scenario or a nantotech diamonding scenario isn't going to go impact Mars.