They made it very clear that while they've held these opinions for a while, they've avoided situations where they would be presented with either lying about their opinions or bad-mouthing the company they work for whether out of respect or fear. I think that's as professional as you can expect people to be. If, as a company, you expect people to not speak about their subjective opinions after they are no longer getting paid as an employee, you need to be willing to offer them something for that, because what you're really asking for is an NDA.
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.
I'm responsible for vetting possible hires to work under me. What this person did falls well within what I would accept as responsible behavior for someone I was considering hiring. The fact that they note they went out of their way to avoid conferences where they would likely be forced to compromise their morals in some way, either through going against the wished of their employer or being untruthful, speaks very well of their character, if it's to be believed as presented. People that have no problem lying for the company, or badmouthing the company to external people while they work there, are both types of people I would desperately try to avoid.
I don't think there's a reason to push the former, as it's got plenty of natural support from the unequal relationship between employers and employees. We may not be able to throw off the shackles entirely, but let's not shame those who rattle them a bit, you know?
Stop mincing words, please.
If it's deemed "unprofessional" to talk about it, then to all intents and purposes -- "if he knows what's good for him" as the saying goes -- he can't.
