There is such a thing as a symptom pool. Some people who are having a difficult time instinctively acquire symptoms that are fashionable and signal their difficulty, but the symptoms are not related to the actual problem. The prototypical example for this, IIRC, is incidence of anorexia in some specific country (Hong Kong?). An anorexia awareness campaign caused that symptom to become fashionable and increased the apparent incidence much more than can be explained as an increased ability to observe the cases that were present before.
This applies to some but not all of the social dark matter being discussed in the OP. Like cosmological dark matter, there is room for debate about whether some instances of social dark matter are real or not.
My reading of the EMDR section was that the patient had panic attacks, did EMDR, then had a panic attack, and then either the patient ran out of money or it was time to write the paper, so we don’t know about the presence or absence of panic attacks after that.
On rereading that section, it is clear that there is no claimed period of time when the patient was observed not to have a panic attack during that period of time. The last panic attack was labelled as “mild”. I didn’t bother to read back to see if any of the other panic attacks were “mild” before success was declared.