In this early demo (Codename: Chicago), the minimize and maximize buttons have been redesigned, but the close button remains the same, and to the left as before.
I wonder where the author got the idea that the [-] button at the top-left was a close icon. It was the "Control Box", a menu icon. AFAIK it's still there, just invisible -- hit alt+space to open it.
Disclaimer: I'm currently unable to test that.