My guess is, that he knows, that his trademark claims will likely not hold up in court and he'd need to spend some money to get to court in the first place.
As such it's easier to try to negotiation something in the backrooms and since that didn't work, try to extort them, and since that failed, try to publicly ruin them, which seems to backfire in a spectacular way.
Automattic has sponsored WP Engine in the past. Matt has talked very good about WP Engine in the past. WP Engine's use of WP is not a violation and WP Engine's use of "Wordpress" is arguably a descriptive usage - at the very least, it's very hard to argue that WP Engine could be confused with Wordpress itself. So even if there's really something to sue over regarding trademark usage, it will be really hard to argue, because of how long the usage has been accepted.
I've been really surprised that wordpress.org isn't under community governance, but seems to be a quasi "charity" project by Matt. It's at the core of the whole community, but a single person holds the key to it? We should really get wordpress.org into the hands of the community and transparently finance it through the foundation.
Personally, I don't think it's right to block anyone from using wordpress.org's theme/plugin/update repository functionality over a dispute with Automattic or personal grudge from Matt.