There are patients who were misdiagnosed with ME/CFS and who turned out to have something else. I completely support looking for an alternative diagnosis if ME/CFS seems like it's wrong.
But there are also a lot of patients with more than one diagnosis. These folks might post saying "turns out I had" when they were just adding information about a new problem, not necessarily overturning their ME/CFS diagnosis.
I did the same google search that was posted. The third link in my results was to a CFS blog where the person is describing how they first got sick:
http://surviving-cfs.blogspot.com/p/about-me.html
"Turns out I had mononucleosis. ..."
So, that's not an example of misdiagnosis as far as I can tell. (I did not read the whole blog)
To give a personal example, it turns out I had endometriosis (tissue normally found in the uterine lining is found growing outside the uterus, can cause pain, infertility, etc.) long before I had ME/CFS. My diagnosis was confirmed by surgery many years ago. In all the different ME and CFS diagnostic criteria that I have read I have never seen endometriosis listed as an exclusionary condition. So, I think it's pretty uncontroversial to say that I have both ME/CFS and endometriosis.
In fact, a woman diagnosed with endometriosis is 100 times more likely to have a diagnosis of CFS compared to other women. (I can dig up the reference if you want it.)
It sure would be nice if there was some sort of a limit or quota, e.g., only one illness per customer.

But there are a lot of us here who have multiple diagnoses on top of ME/CFS.
Often there is a proven correlation (e.g., the well known overlap between ME/CFS and OI). But that does not mean we know the cause. Does one illness cause one or more of the other illnesses? Or does some unknown underlying issue cause two or three correlated illnesses? Or what? It's complicated.