Without additional context the letter does read as persuasive. Is there significant additional context? Having looked at Matt's comments in the speech I'm not seeing any actual substance of what's wrong with WP Engine.
It’s kind of perverse of Matt considering: A. He accuses “WP Engine” for being confusing branding. He literally owns WordPress.com; which confuses tens of thousands of people on a daily basis. (“Are you on the WordPress login page?” “I swear that I am!”) B. He complains about the post revisions not being limitless. But until recently, WordPress.com had a limit of 25. C. If post revisions matter, surely plugins matter…
Edit: To your first point, Automattic, who originally registered the trademark, apparently has a license from the trademark owner (the Foundation) to use the mark (at least for that domain). https://wordpress.org/book/2015/11/the-wordpress-foundation/ WP Engine, by their own admission, does not have a license. It also seems odd to call Matt perverse in what seems to be a trademark dispute without any acknowledgment that he is the inventor of the software, as such the founder of the community, that his friend Christine Tremoulet coined the name, and that his company originally registered the trademark.