Whether legal or not, it's certainly childish. The fact is anyone could have seen that this was a disgruntled former employee so there wasn't a need to say anything at all. If they had to comment then keep it brief. However, getting into a flame war on reddit (even if you are the CEO of the site) just makes the whole company look bad. Heads of companies simple do not get involved in pissing matches with former employees like this without a hit to their own reputation.
This is where the post goes horribly wrong, too much detail. If the former employee wants to start a lawsuit, any detail that can not be defined, was not measured, or is not written down is a liability. For example "not getting much work done"
People were starting to question whether Reddit had a good CEO and corprate culture already, now its certain to gain momentum.
A lot of people have a natural tendency to always side with the underdog regardless of the facts of the situation, especially when it's one guy vs. any sort of institution (a company, school, whatever). That probably says something nice about humanity, but it doesn't make it any truthier.
How did you jump to the conclusion the CEO's version "was just the facts"? You seem to be saying because he is the boss that he is being factual and the employee inherently was not.
The CEO's spin might be the facts but it would be impossible for anyone who wasn't there to know. That's why an employer wouldn't normally air stuff like this in public, its one person's word against another, and the liability risks are steep.
I'm not basing my comments on which one of the two parties was being truthful, they might both think they are. You would have to do an indepth analysis of the employees performance, and the circumstances of his termination to know for sure.
I'm saying it was really poor judgement for a powerful employer to unload on a relateively powerless former employee, and air what should have been confidential HR information in public like that. If I were applying for a job with the guy I would think twice about working for someone willing to savage an employee in public like that. If I was on his board I'd sure think twice about whether he is the right person for a CEO job.
Even giving references can be an issue. The law states that the company must make an effort to make sure the information is accurate and complete. For subjective measurements, there may be argument over what is accurate. When my father was working, their lawyers recommended they not give any information other than confirming employment dates and duties for references.