Sorry to break the news ( I could be wrong) but I google-fu’d the hell out of this when I got my Cover. Apparently Microsoft didn’t see the point in adding this option to Windows 10 Tablet mode, they just assumed you’d be disconnecting the keyboard, not folding it back.
I believe only the Lenovo 2-in-1 tablets have this feature, because Lenovo added it so that it disables the keyboard when its flipped back, but that’s probably because the keyboard is not detachable.