Yes. Windows Shell, Xbox, Office, Skype, HoloLens, etc. are all using UWP.
The Office version you get with Office 365 ("Desktop Office"), the one, businesses use, is certainly not UWP.
Skype UWP is a preview announced a few days ago.
"Are all using UWP"? No. They are in a transition of moving to UWP, but every serious business application is Win32.
It's a hybrid: start menu, action centre and some other elements are UWP's XAML frames now.