In the spirit of non-disparagement, it seems like Yishan could have sent a private message to the employee which reiterated why he had been fired, and asked him to stop posting about it. If Yishan felt that it was necessary to comment, he could have still posted something without Reddit losing face – something like, "We're sorry that you feel our decision to let you go was unclear. I've sent you a private message, and would be happy to go over things again with you personally."
I don't think that is the case. I certainly would respect the CEO stating clearly that the employee was fired, that it had nothing to do with criticizing the lack of charity, and the reasoning has been clarified in private.
More like "Reddit to lay off an employee over feedback on charity spending"
Don't forget that just before Reddit's CEO commented, this guy was a hero of the thread; his comments could have started a completely different discussion.
As far as looking at all sides goes, how do you know he didn't do that already? I'm not sure what the culture around letting people go is at Reddit, but I'm inclined to think he knows perfectly well that he was fired, and why, and is at best shading the truth in his favor to make himself look better.
The best thing to have said? Really, nothing at all. There was no reason that anyone in the company needed to chime in and acknowledge (let alone contradict) what this guy was saying.
Why should he say anything? Do you think that a bank CEO would interrupt his day to comment on the termination of an analyst?
If you need to deny it, you can say, "This isn't an accurate account of events" and leave it at that. But even that's unnecessary.
Sure, this guy was phenomenally stupid to start a pissing contest with his own employer on a forum said employer controls but that whole pig/mud thing is really true.