Simply put, I believe that while good intentioned, the “no personal medical advice” rule is doing more harm than good.
You have to remember that people come here from Google. If SE isn’t answering these questions, they’ll keep going on through Google until they find something that seems to fit - and in so doing, they’ll get the advice that you refused to provide. The link given has an example of someone asking about fatigue and then suffering serious consequences because they drank a lot of water and exercised - now consider for a moment that that patient is likely coming here from Googling “fatigue low energy” or something, and if they don’t get an answer, they’ll go to the next site on the list - which might give the exact same harmful generic advice that you’re concerned about, or it might give pseudoscience “alternative medicine” advice that actively harms them in other ways.
By contrast, if Answers for these questions are given here, then we can be sure that they are high quality answers given by medical professionals - even if they might not be as good as an in-person examination, then they’re still better than going onto the next site that may just be a medical listicle and not consider their personal circumstances at all. Additionally, while the patient may not give a full accounting of their medical history, that’s what the comments are for - to ask questions for additional information that can help produce a higher quality answer. If someone posts “I have a lot of fatigue” without enough detail, then you should use the comments to ask them “Do you have conditions X, Y, or Z”?
As a result of these factors, I believe that the current policy is counterproductive to patient health and should be changed. Instead of the current policy, I would suggest adding a note in the sidebar that says something like “This site is not a replacement for seeing your doctor”, and maybe putting a banner on personal medical advice questions that advises people who find them through Google to consider if it matches their circumstances- and to ask a question of their own if it doesn’t.
"By contrast, if Answers for these questions are given here, then we can be sure that they are high quality answers given by medical professionals"-- How so? There are several high-rep users here I believe to be licensed physicians, but I have absolutely no way of verifying that. They could be brilliant 14 year olds for all I know. Stackexchange doesn't guarantee credentials. It only provides upvotes and downvotes.