Very juvenile and unprofessional way of dealing with the situation, really erodes trust in the platform (simply deleting the comment would have been a better response).
Would maybe expect this from the founder of a young fledgling startup, but the 33 year old CEO of a company like Reddit ought to know better.
If I read this Reddit thread without knowing him, I would have deemed him unprofessional and maybe even upvoted some of the comments.
When I read the thread knowing who he is, I'm thinking "I can't imagine how stressful it must be to run Reddit. He made one mistake in a bad day, apologized for it, and now everyone's talking about it. Steve's way nicer and more professional than I am, so I would probably have messed up big time in his shoes."
You comment that he's made just one mistake on a bad day. Perhaps this is just his most visible mistake, and he's been making these kinds of bad mistakes for the past month. It didn't take him 10 seconds to do this - he had to log in with full access to the reddit database and run unprotected queries against the live running copy. That's both shocking security, operations and basic common practice. For a childish insult.
And finally, he did not actually apologize for any of this. "I fucked up" is not the same as "I'm sorry".