> I wonder if you could use a forum post of someone saying what's wrong on their page as a source to edit the page though.
You could. The person themselves couldn't. (There are guidelines about the risk of bias from primary sources, which apply very much to someone talking about themselves or their own company, so you shouldn't just blindly copy what they say into Wikipedia, but you absolutely can use them as a source)
Off-topic: this discussion reminds me a bit of what happened about five years ago when I offered to host a reddit AMA about anesthesia and anesthesiology and related subjects (I was board-certified in 1980).
The moderator asked me for proof that I was who I was, so I had to spend a couple days digging through my files to find my medical school diploma, board-certification document, medical licenses, etc., then scan and email them to reddit.
I did all that, and they said it wasn't sufficient proof: I needed to take a selfie with the documents and my driver's license to "prove" my bona fides.
You could. The person themselves couldn't. (There are guidelines about the risk of bias from primary sources, which apply very much to someone talking about themselves or their own company, so you shouldn't just blindly copy what they say into Wikipedia, but you absolutely can use them as a source)