I interpreted that to mean the opposite: an inability to leave your planet means you have little resilience to planetary-scale destructive events, and no resilience to planetary destruction. Alternatively, we're less likely to identify or recognize an intelligent civilization on a single planet, as opposed to one across multiple planets or systems.

I wonder if being multiplanetary actually makes a civilization more vulnerable rather than less? The thing about nuclear war on Earth is that it's basically unwinnable because of mutually assured destruction, and the threat of nuclear winter. Even if you win, you lose.

On the other hand, it might be a lot more possible for a civilization on one planet to win a nuclear war against a civilization on another planet. Same with bioweapons: on Earth it would be stupid to try to use an "enhanced" ebola or COVID or bird flu as a weapon because you'd infect your own people as well as your adversary. But if you're separated by a great distance, that's less of a concern.

In a way, the "dark forest" interpretation is too optimistic. The assumption is that one species will always try to destroy another independent alien species if they find one. But I wonder if it's even worse among members of the /same/ species but different cultures that are separated by a great distance.

Because additional grams in orbit are equivalent to additional joules of energy released as weaponry, e.g. dropping rocks. I fear that our ability to use space to destroy ourselves will come before we can use it to launch a diaspora.

Doesn’t rock dropping as a weapon seem a bit redundant when we already have the ability to make lots of 20+ megaton fusion bombs? I guess non-proliferation is harder.

> I fear that our ability to use space to destroy ourselves will come before we can use it to launch a diaspora.

Will come? It is here. Have you not heard about intercontinental ballistic missiles?

In that case I have this bad news for you. Multiple actors around the globe maintain the capability to fling warheads at others (through space).

Not sure what you mean by “destroy ourselves”, but our capability of destruction is not constrained by cost/mass in orbit.