His “member since” date is the date on his homepage, like everyone else. If he wanted a different date, then he could have changed it at any time.
I guess that’s when that feature became active or something. According to Grok and a couple articles the site actually became active in 1996, which really was pretty early. I recall getting a browser on my work computer in 96, but not being able to find anything or make any use of it until early 97.