People can talk about it whenever they want, I'm only speaking in support of the very narrow point that it is unprofessional. People reading the post will use its contents to form (or modify) their opinions of the author, and I believe it will generally be in the negative direction. Publishing something that has a net-negative impact on your professional reputation strikes me as unprofessional in a very basic way.
Can he do it? Sure. Should he? Maybe, depends on his values. Hopefully he understood how the message would be received and is OK with the possible consequences, and valued self-expression and making an authoritative critique of MySQL more highly than the negative impacts of rubbing a few readers the wrong way as they perceive a breach of generally accepted professional decorum.
Because he is stirring things up over there right after he walked away so it won’t be affecting him. The post is a big warning sign to anyone considering working with him.
Can he do it? Sure. Should he? Maybe, depends on his values. Hopefully he understood how the message would be received and is OK with the possible consequences, and valued self-expression and making an authoritative critique of MySQL more highly than the negative impacts of rubbing a few readers the wrong way as they perceive a breach of generally accepted professional decorum.