>is this is a public letter, the intended audience is everyone
Just because something is public does not mean the intended audience is also everyone. Something can be intended for a subset of people, but broadly published. But I think this has been hashed out somewhere else in the thread.
I think "management knows what they did, don't believe a word they say, I'm out, peace!" is a bit of a disingenuous reading of the letter, but you have a valid point about the last paragraph and I'll walk back what I said a little bit.
When I resign from a job, I send my resignation to my employer in private. I wouldn't put anything in it that I'm not comfortable with becoming public, and if they wish to share what I wrote that's fine, but the purpose of being discreet is to leave the level of disclosure up to the other party's discretion. You don't send a company wide email saying "management knows what they did, don't believe a word they say, I'm out, peace!"