If the central repository is a problem, then each site draws on it in proportion to their usage and it isn't obvious why one site is an issue and others aren't.
Hence my question, does he wish hosts to move away from a central repository? He certainly can advocate for that but it's a larger issue than WPEngine.
Also sort of adjacent but Matthew Prince of Cloudfare has just offered to donate resources to fund the whole central repository: https://x.com/eastdakota/status/1841154152006627663
The norms of open source software don't really have much to say about this. Mullenweg is right: the typical thing companies that run GPL projects do when they end up competing with firms that don't pay their freight is to relicense. Being a little ruthless with trademarks seems strictly superior to switching to a "source available" license.
I do hope they open source the API. I might take a stab at implementing it using wpackagist (though possibly that's what they're using to mirror already)