The continuation of the sentence you quoted is "and there will also be new cooperative strategies."

This is not faith, but an observation of Earth's past. If it were the case that intelligence is itself harmful, then it should not have evolved in so many species.

And even if we look at only humans, we do not see dumber populations being more prosperous. One could argue about the definition of "dumber" and "prosperous", but it is at least true in my own judgement of value, which I suspect is not too far from the average in modern society. One could also argue that bio/cyber adversarial games are qualitatively different from all past adversarial games, but you could argue the same thing for other qualitatively different games that emerged in the past, when they were new. But we're still here. So this argument hinges on burden of proof, and I think that's on Anthropic.

Also, as mentioned in that first linked comment, centralizing AI development increases a different kind of speciation risk: a small group of superintelligent humans, likely the ones currently running the top AI companies, splitting from the rest of humanity.