It only took 5+ minutes of poking around to figure out 1) it's not for MacOS, it's for Mac OSX, and 2) it's something different than the existing Cocoa OSX support that's apparently been around forever (since Emacs 23, apparently), but never seems to get around to clearly spelling out what that is.
I think the money quote is:
So if it [the Cocoa port]is good enough for you, then you don't need to try this.
OK