I mean, it's arbitrary, but it's not like it's a contradiction. In both instances, you
can start with an assumption of mutual interest in positive-sum interactions, and still end up with a universal threat-assumption.
(And it's also kind of definitional to the meaning of "positive-sum." A positive-sum interaction is better than no interaction. Insofar as a civilization is optimizing for... basically anything, it would prefer positive-sum trade [from which it acquires resources, information, etc] to no trade. At the very least, all else being equal, the resources and information would increase the civilization's odds of survival.)
Let's assume that the vast majority of alien species would like to have positive-sum interactions with other alien civilizations, if that were possible. But they can't assume a guarantee that there isn't at least one civilization that defects into being predators, and would come to destroy them (and any other civilization they could discover through them) if they caught that predatory civilization's attention.
As such, the civilization goes silent, hiding from such predators; and, as such, the civilization immediately punishes any other civilization that may reach out to them, trying to "shut them up" before that other civilization's directed communications reveal their own location. Which means that, in effect, due to simply being aware of the existence of the possibility of such predators, every civilization becomes the very predators they're imagining.
And because every alien civilization can work this out, every civilization can conclude that even if there weren't predators at first, the equilibrium state is for everyone who wasn't a level-1 predator to have become this type of level-2 predator.
(And yes, there is a social-network equivalent of the level-2 predators — these are the "cringe reaction" accounts that get attention by punishing the violations of the performative-perfection norm.)
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Or, to be formal about it: the dark forest hypothesis is essentially timeless decision theory applied to the game-theoretic tit-for-tat strategy. The same logic that argues that Roko's basilisk can force you to enable its existence before it exists to enforce that, argues that the structure of "the lawless cosmic void"-as-social-network can force your own civilization into choosing "defect" over "cooperate" before you ever actually meet any aliens who could enforce that. Even if your civilization really wants to choose "cooperate"!