I have both Start8 and ModernMix installed on Windows 8.1 and it appears to be configured to return to the Windows 8 menu upon closing a modern UI app. However, every time I close a modern UI app, it returns to the desktop.
I thought the problem might be with ModernMix so I uninstalled it, but every time I closed a modern UI app (by dragging it to the bottom of the screen), it still went to back to the desktop, so that didn't appear to be the problem.
I am running Start8 1.31 and ModernMix 1.12. ModernMix is set to return to Windows 8 menu when I close full screen applications. So far as I can tell, there is no control in Start8 relating to this behavior.