I think it's that "in the works" seems to imply an agreement/commitment, but this appears to be a proposal.
However, I think we all agree we want the Thunderbird replacement to be a desktop client (plus other platforms like mobile), and based on web technologies, most importantly written in JavaScript.
But the title has now been changed and that is fine with me.
[0]https://mail.mozilla.org/pipermail/tb-planning/2017-April/00...