Its an interesting theory. IMO: the difficulty of destroying a civilization is probably harder than communicating with them, and the advantages of keeping one around probably outweigh the advantages of killing them if you're already more technically advanced.

A big point in TDF is that the speed of light limits our ability to communicate and understand other civilizations. But, by extension, weapons are also limited by that limit. Of course, maybe its possible to break the speed of light, but then communication could also break this limit.

So, if you're a technologically advanced alien civilization and you become aware of humanity, you can send an Envoy or you can send some Nukes (or, of course, you can do nothing and watch). Just within the context of The Dark Forest Theory, there's practically no reason to send Nukes; at worst, you discover that humanity sucks and then you go ahead and nuke us. But life is probably special; maybe humanity would make great trading partners, maybe our brains function differently and can solve problems differently, maybe our natural proclivity for war would make us great allies, or maybe we'd make great slaves for the mines on the Vartoth 12 colony. All of these are options for a civilization a thousand years ahead of us; they can pick what they'd like, but none of them would classify as a result of The Dark Forest theory.

There's also a possibility that maybe this hypothetical alien civilization tried that course of action in the past with another civilization, and they were friends for a while, helped each other, then fought in a bloody war, and now that civilization is distrustful so they destroy any young civilizations while they still easily can instead of even trying to come to terms. But that's not a Dark Forest theory situation; the Dark Forest Theory suggests that its natural for civilizations to want to kill each other off.