It may not be. If you're happy with ProseMirror, continue to use ProseMirror. I have your back.
But as the blog posts describes, I had a whole pile of new design insights that avoid some of the issues I've ran into with ProseMirror, so I had the itch to build a new iteration.
I'll add a link to the blog post to the docs section of the website.
(It's marijn, not merijn.)