I've seen other research like this - trying to warn participants about any potential risk of the research can end up sounding a bit silly in some cases.
The bit about risk may be a standard thing ethics committees require researchers to assess and state for participants taking part in the research .even if the research only requires participants to fill in a questionnaire like this one.
I took that to mean that the survey may trigger PTSD symptoms that are caused by traumatic experiences in their career / professional development. I think this would especially pertain to researchers that also have ME (or family member). I suspect the same information about recall potentially causing distress, would also be used when asking ME patients to recall their experiences with the health care system. Most (all?) ME patients will have at least one distressing / traumatic event from physicians / health care. (Like Dr Klimas said, but I'm paraphrasing: 'They came to the doctor with ME. They left with PTSD.')