Wikipedia policy discourages entities from editing their own page.
Forum posts aren't outside the realm of valid sources, so where exactly is the line drawn?
That said, as far as where the line is drawn for sources see https://en.wikipedia.org/wiki/Wikipedia:Reliable_sources i dont think forum posts are usually considered the best sources.
But, of course, none of those posts specifically targeted their Wikipedia articles.
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)
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.
At that point I said WTF? and bagged it.
No good deed goes unpunished.