1. He directly manipulated the production db (seems like they use Cassandra). Definitely doable for someone with a deep technical background, even if they spent 99% of the the past couple of years working "via other people", rather than writing the code themselves. I could easily imagine him getting database access privileges as a part of the "catching up what's happened since I left" process.
2. Reddit has built easy-to-use, e.g. web based in-house tools for editing comments - without leaving any trace of the fact that they were edited.
I guess there also a third option; that he fudged the truth (err, lied) when confessing: that he asked some developer to do the work for him: presumably while laughing and thinking they were hilarious, and then took the blame for when it was time to confess.)
I think he needs to resign. Such a dumb, irresponsible action. I don't think someone that impulsive and immature should be the the CEO of Reddit. And since he's 33 rather than 23, it's not very likely he'll much more mature within a few years.
I'd imagine comments are stored in plaintext in a table and could be edited by anyone with a modicum of tech experience
The CEO didn't just edit the comments, he did so in a way that did not reveal the comment had ever been edited.