They weren't even planning to create a fork.
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Matt said his company was going to all but stop contributing to WordPress for now out of spite against WPEngine https://automattic.com/2025/01/09/aligning-automattics-sponsored-contributions-to-wordpress/
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These people said "OK we'll take responsibility for the next release within the current WordPress organization framework so that work can continue"
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Matt said nuh-nuh, no you aren't https://wordpress.org/news/2025/01/jkpress/
Basically Matt interpreted their "we're going to take over work within Wordpress since Matt abandoned it" as "we're taking over Wordpress from Matt" and is telling them through some tortured rhetoric "you aren't Wordpress, I'm Wordpress, you go do it in a fork instead"