Well, there are many existing and more complete solutions to "callback hell", many JS devs worth their salt already use futures/promises/defers in one way or another. The author seems to ignore existing solutions completely.
Whether this project will even be finished -- or much less, be better than other solutions -- is yet to be seen. Any features that make Elm's callback solution stand out from the crowd are not being advertised.
i've been following elm with interest for a while now, and the author's general blogging style is "here are some reasons you'll be happy using elm" rather than "here is why elm is better than all the other altjs languages out there". elm doesn't need to stand out, it just needs to be a pleasant, productive way to do things.
Whether this project will even be finished -- or much less, be better than other solutions -- is yet to be seen. Any features that make Elm's callback solution stand out from the crowd are not being advertised.