Dude, it's the same guy. I'd bet a medical school acceptance on it. I'm a moderator on a couple of other forums, and no less than ten times have I seen someone make a stupid comment, try to drag it out, and then when they realize how stupid they sound, say that somebody else logged in to their computer. Usually it's a brother, or friend, or they were using a public computer.
For what it's worth, I left myself logged in to my school library once, and someone totally pawned me. What they did not do was go on to SDN and make silly posts that almost sound sincere, but are just stupid. Especially since everyone on SDN is anonymous and it wouldn't have been a good troll. What they did do is go on to my Facebook, change my profile picture, send messages proclaiming gay love to all of my friends, change my status to gay porn, etc. THAT is what people do when you leave your computer up.
No, the OP is still the OP, and just isn't willing to take responsibility for making a dumb comment. Guaranteed.