People are allowed to be blunt and even curmudgeon, there's nothing wrong with that, it's just one of the many dispositions we can assume. That's diversity. This is a personal blog and it's totally reasonable the he takes this posture, it's slightly refreshing. If this were a public intellectual being taken seriously, or if he had a huge audience, then there would be more scrutiny, but otherwise he's just a guy with an odd way of communicating with a small soap-box on a small corner. It's fine.
These concept are pretty much orthogonal. While I don't disagree that insistence on politeness can get in the way of good discourse, it is an all-to-common mistake to read or promote lack of politeness as a signal of honesty and accuracy - which manifestly isn't the case.
It's also often done in a lazy way. Which doesn't imply the thinking is also lazy, but there is a good correlation.
Your sentiment is a really common and insidious one that way too many people use to feel smug and "rational" about being mean