Almost nothing on the internet is truly anonymous. Nearly everything is traceable, and erasing all the traces that might lead back to you turns out to be pretty hard. Basically, if you wouldn't say something in a crowded coffee shop, you probably shouldn't say it online, unless you take extraordinary precautions. This particularly applies to unsettlingly detailed accounts of your life as a serial rapist. This user might think he's anonymous, but there's not much stopping a court from using his post to throw him in jail.
Here's how it would work.
Whoever left the comment used a dummy account, but Reddit still knows everything about the guy, including his IP address. Under threat of subpoena, they'd have no choice but to share that information with law enforcement. It's happened before.
More importantly, everything the poster said is completely admissible in court. It's technically hearsay, as if Reddit were a witness recounting what the defendant said, but hearsay is allowed in court under exceptions for "party admissions" (statements allegedly made by someone who is party to the case) and "statements against penal interest" (statements that contradict what the speaker is arguing in court). According to University of Maryland law professor Amanda Pustilnik, "It's as if I said in a Facebook status update, 'I cheated on my taxes this year.'" Criminals give themselves away on Facebook all the time (here's one from this weekend), and all of the same laws apply to supposedly anonymous Reddit confessions.
The poster could always claim his account was hacked, but that wouldn't be enough to keep it from counting as evidence. As Pustilnik told us, "The prosecution doesn't need to prove authorship/identity to a 100% certainty. It needs to lay a foundation (using other evidence) such that a jury could reasonably conclude that the accused is the author."
Prosecution is nonetheless unlikely. A DA could try and pinpoint the time and place the crimes allegedly occurred and then use publicity to try to get victims to come forward, but that would require a significant expense of resources on a case that might not even exist, and prosecutors' budgets are not infinite. A victim might recognize herself in some detail from the post and press charges, but that hasn't happened yet. If a victim were located, though, she'd have a pretty strong case. Reddit being Reddit, there's a decent chance this is all a hoax, but if it's real, it's probably the riskiest post they've ever hosted.
Many thanks to the University of Maryland's Danielle Citron, who also provided legal expertise for this post.