Enjoying the drama but I'm surprised that he isn't under a stricter confidentiality agreement.
Does this happen often? (where an ex C-level executive gets to publicly speak against his former company without repercussion)
It doesn't matter if you're C-level or not. As soon as you get to a point in your life where you don't care if you ever work again you can say pretty much anything you want.
Not to mention that, if what has been said is true about reddit struggling with profitability, he probably has little to worry about in terms of a suit.
I don't believe he has actually revealed anything that would be considered "confidential". The thing is most ex C-level executives don't act this way because they don't want to look bad to their current employer, or ruin their chances with future employers. Burning bridges like this is usually not a good idea if you want your career to keep moving forward.