I don't know enough about MacOS GUI development, but I'm guessing if an app requests a full screen display, it gets told the resolution is the resolution minus the top, and that's just blacked out.
I guess sometime next year I'll get to find out if Illustrator actually requests Full Screen Display or if it just throws up a window with no chrome that covers the menu bar!
They hinted at this with some of the screen shots they showed:
https://imgur.com/a/McNBTHY
I don't know enough about MacOS GUI development, but I'm guessing if an app requests a full screen display, it gets told the resolution is the resolution minus the top, and that's just blacked out.