What are the harms of self diagnosis? We already KNOW that certain "disorders" are under diagnosed in adults. We also know that the demographics of the self-diagnosed population matches that of the expected undiagnosed population.
Screening tests have difficulty distinguishing self-dx individuals from those with an official diagnosis. What is the harm in welcoming self-dx individuals into a community? You aren't 'cheapening' or 'weakening' a condition but acknowledging people with shared experiences.
What if a self diagnosed person is wrong? Then you've just gained an ally, an advocate, somebody willing to listen, somebody likely with an adjacent condition that will work with you towards joint understanding or advocacy.
Diagnosis is a priveledge. It requires time and money and subjects you to the clinicians biases about who can and can't have a condition. "You can't be Autistic because you are in a relationship". "You're too bright to have ADHD" "His lack of eye contact is due to anxiety"
Often times there are unspoken biases: "You can't be Autistic because you're a woman", "You can't have ADHD because you're an adult", "EDS is very rare so you probably don't have it"
At the end of the day, if you (1) experience all the same symptoms as people in a given community, (2) are willing to listen and learn from people in that community, and (3) get value from being in that community (and contribute to it) then you should not be arbitrarily excluded.
Diagnosis is based on luck - what area you live in, who the nearby doctors are, how good they are, what biases they have. And that all assumes you have the time and money to seek diagnosis and you don't get shut down by your GP. Even seeking diagnosis can also be stigmatizing.
I have yet to hear a valid argument against self diagnosis. I'm sure there are some, but that won't change the fact that the benefits (to both sides) outweigh the costs.
I don't suffer through all my issues every minute of every day just because it's "cool" or a "fad". I don't get to turn it off because I get sick of it. It is who I am and it defines me. Diagnosis labels are arbitrary and change every 10 years or so, where my experience will
always be my experience. You can call it whatever you want but you cannot deny it to me.