One takes a rather different tone in a pointed and deliberate "fuck you; strong letter to follow" than in a resignation letter that's mostly polite BS, as is typical.
I'm not sure the point was about tone. It was letter by an academic for an academic touting the former's intellectual contributions and apparently intended to be publicly displayed. I would have expected something more structured, easier to follow, and better targeted.
It does seem rather inside-baseball for something released publicly. Maybe the audience is only a subset of the public, though, and this was just the simplest way to get it to them.