There's a bit of a tradition of this sort of thing. The classic is this one, of course: https://news.lettersofnote.com/p/very-truly-yours
But there are many more; some more modern examples: https://abovethelaw.com/2014/07/the-greatest-letter-ever-the...
It's... one way to do things; most lawyers don't write letters like this, but it's hardly unprecedented.