The former employee is not the issue here. While reading through most of his AMA he seems pretty fair, respectful and thoughtful in his answers and he's entitled to his opinion, it doesn't discount him from having been a terrible employee. I'm not defending him in any way shape or form, but you'll notice he was very careful with how he worded his statements.
The problem is that as a CEO you never do this. There's a lot of very good reasons why. At the start of that list is "it's unprofessional and bad form", at the bottom is "you'll get your company sued for libel" and in the middle are a whole long list of reasons like "damages current employee morale".
As much fun as it is seeing what appears to be an ungrateful shiftless employee get his comeuppance, airing dirty laundry like that as the CEO of a company is an absolute and very basic no no.
We, as the public, don't even know if what the CEO was claiming is true. If it's not, he just opened his company, now fat with cash, up to a very nice libel lawsuit. If it is true, he may have just irreparably harmed the career of a young tech worker at little consequence to himself, that's just bullying and really bad form.
If what the ex-employee said is true, he was fired for questioning a company policy, I'm sure there's a lot more to the story than that. He could have become disruptive over his questioning and the decision was made to terminate his employment. I don't think most people would have any issue with that, lesson learned on all sides, move on. If it's not true, it's not really all that big a deal, I'm not buying stocks based on his say-so anyways. People make all kinds of claims about their own employment all the time.
The CEO shouldn't have said anything at all. If he felt he really needed to, a simple "Yes, we terminated his employment, those are not the reasons, the reasons are not public" and leave it at that.