Any metro applications launched from Start8 is included as part of the MFU list. We have less information on them due to their nature (just launches).
Launches from the Windows 8 start menu would not be counted I think.
Start8 is written in C++.
Actually, launches from the Windows 8 start menu are counted. I never used the "Modern UI applications" folder from Start8 to launch metro apps, only used the Windows 8 start page, but all the apps I launched would always show up in the MFU list. This is why I was so happy when the option to hide Metro apps from Start8 was put in a few versions back.