The graphical Emacs backend, unlike the rest of the things you mention, was not developed by one of the "regular Haiku developers" but by one of the Emacs devel

The graphical Emacs backend, unlike the rest of the things you mention, was not developed by one of the "regular Haiku developers" but by one of the Emacs developers, and fully upstream, too, which makes it doubly exciting. Indeed it's now available in Haiku's package repositories (or of course you can build Emacs from source yourself.) :)