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.
Emacs-MacPort is an old port using the Carbon API, which has long been deprecated by Apple.
But to be fair, Emacs-NSPort (the official, Cocoa Emacs) is sorely lacking in maintenance power, and has a lot of problems, including tearing, glitches, crashes... It does not even have an official maintainer! [1]
OTOH, The MacPort has a maintainer, has more Mac-specific features, and is more stable. That being said, I still use the official port (due to various reasons).
[1] Try to look for src/nsterm.m in admin/MAINTAINERS.
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