I don't know who he is, but he's not Paul Graham. The story behind that is that pg emailed me on April Fool's asking me to help him with a hoax to make it look like he was really nickb, who was the most prolific contributor on the site at the time. pg just manually changed the account name on a reply to make it look like he was accidentally replying under the wrong username, and my job was to submit a story looking like I had discovered this.
(This was also already publicly discussed somewhere on HN previously, albeit several years ago.)
It's now part of "ancHNt history" (nickb hasn't posted for 6+ years). I did recall some discussion about nickb = pg, but don't think I had seen the 'smoking gun'. Noting how Reddit was started, I'm neither surprised nor concerned if there was such an account in the early days (either by pg or by the yc partners).
[Edit] Dammit. Now I've read Alex3917's response, I wish I'd done the accurate calculations on that "Smoking Gun" link to note it was posted on 1 April, 2008.
(This was also already publicly discussed somewhere on HN previously, albeit several years ago.)