We all know it's impossible if just guessing right? That player might even retired from minecraft
Well, there's only two way to find out.
Server or player log or something like that. But only some people with the authority that are able to access it, not to mention if there's changes as the server is pretty old ( reset, deleted, lost, corrupted file, etc). And we don't even know if the log record the first join time or list the players down based on first join time.
Or the first admins or owners still remember or have a kind of documentation about the first player ever.