The CEO didn't just edit the comments, he did so in a way that did not reveal the comment had ever been edited.
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
I strongly get the feeling that tools were created at Reddit to allow them to do this, suggesting that sometime ago employees at Reddit thought "wouldn't it be great if we could change what users actually said"...
Don't get me wrong I feel no protectionism for a lot of the scumbags on boards like Reddit, but I would find an extreme amount of hypocrisy in "the front page of the internet" actively manipulating its content in such a way.