You can observe patterns of behavior, develop theories understanding, attempt/experiment with interactions, and refine based on the results. That's communication (and doesn't assume anything about the other alien civilization).

Now, civilizations may be more or less willing to do this and more or less successful, but that's not the same thing as no one will dare try, as the dark forest theory wants.

(Personally, I think civilizations that are better at this will outcompete ones that are worse or refuse, though that's just my own opinion.)

> Bold assumption that they are self limiting.

Name the exponential phenomena that aren't self limiting -- that don't consume the medium which allows them to exist in the first place.

> I think the answer tends to be resources, which is the foundation of dark forest argument theory to begin with.

Well, yes. One of the reasons the dark forest theory isn't coherent.

> Any real alien reasons would be alien to us.

Yes, but this doesn't back up the dark forest theory. It also doesn't mean aliens cannot be understood at any level or interacted with in any way.

(The dark forest theory makes very strong claims on the logic, intentions, strategies, resource use/governance of alien civilizations, BTW, and wants this to be uniform amongst them... even though the one civilization we actually know of doesn't adhere to them.)