I do agree with your probability assessment; I too think that the Prime Directive is a more plausible answer to the Fermi paradox, as is “we're just alone”.
However, it is not necessarily the case that everyone suddenly gets talking as soon as one of them breaks radio silence. If everyone is silent because of a perceived possible threat, then it stands to reason they will continue to be silent even when they receive a message.
Humans have shown time and time again in their history that they are expansionist, imperialistic assholes. For aliens to be spooked, we only need to show that tendency on a cosmic scale. If we start sending out probes or even manned missions to far away places that are barely likely to support life, we will instantly label ourselves as greedy conquerors. As soon as a colony of ours starts sending out new missions to create even more colonies, that's when all civilizations, even the less advanced, realize that we're the first stage of a cosmic cancer that must be nipped in the bud at all costs.
The oppressive existential threat you reference doesn't need to exist for the universe to be a dark forest universe. It's enough for every sufficiently advanced civilization to realize that such a threat could exist and remain quiet and hidden just in case.