Automattic doesn't own WordPress, for two reasons:
1. WordPress is a fork of b2/cafelog, developed by Michel Valdrighi, and that fork was made under GPL terms
2. WordPress does not have a contributor license agreement or other policy that would require licensing or assigning ownership of upstream contributions to Automattic in a way that would allow them to shirk their responsibilities under the GPL
They're betting on those bankrupting lawsuits never coming because the plaintiffs can't afford to front them. That's a common risk calculation made by private equity owned companies - of which Automattic is also one[0], they're owned by BlackRock.
Automattic does not have a monopoly over WordPress hosting. They bullied several of the major players in the managed WordPress space into paying trademark licensing fees. WP Engine, being exactly the kind of skinflint operation Matt accuses them of, didn't pay up, but they're absolutely going to stick around. The worst thing Matt can do is extract damages and demand a rebrand (since, well, "WP Engine" really does sound like "WordPress Engine"). He can't legally prevent anyone from hosting WordPress.
[0] Yes, every accusation really is a confession.
I think he was threatened by WPEngine’s approach as a more popular offering than Wordpress.com and decided to go scorched-earth on them instead of trying compete on the merits of their product. He’s threatening anyone that tries to build bespoke hosting with extra tools like ACF.