He is now annoyed that someone else has been better at extracting profit from something that - going by what he says - he sees as his personal fiefdom.
I really dislike WP Engine because they ruined Flywheel, one of the best companies I’ve ever dealt with (and to which I paid tens of thousands of dollars over my lifetime as a customer of Flywheel).
But Mullenweg is coming off as completely unhinged.
This is quite inaccurate. Sure, WP started as a fork of b2, but it's not true to say Mullenweg is a cuckoo. WordPress is something he personally did extensive development work on to evolve it to where it is today, and he hired many of the people who did most of the rest of it as it became commercially viable. Even early on it was a quite different product to b2, which was at best fledgling, and it is fully fair to say that he is one of its creators. He wrote loads of it at the beginning; it's his thing as much as it is any other developer's, if not more. We should not diminish that achievement by pretending he is just leeching off something that in fact he substantially built.
Now, whether he is cuckoo is another matter; as you say, he appears unhinged. Something has happened to him such that the more self-absorbed tendencies that used to work quite well in a BFDL context have gone very wrong. He always used to be able to come across as the guy who could help sell this so it will all work for everyone in the ecosystem commercially, and could be likeable and encouraging as a community figure, but something has broken.
I am sad for him because this kind of loss of control is ultimately humiliating him. It's time to take off all (or all but one) of the hats, and find something else in life.
You are right about WP Engine: I am no fan having had considerably less than optimal customer service experiences with them.
But this is fucked up.