Aha!
Terminolgy, terminolgy, terminology:
z/OS is the z/OS operating system.
z/Linux is Linux, the flavor of the operating system that runs on z/Architecture
z/Architecture is the architecture of the physical box. You can run several operating systems on it just like you can run different operating systems on your Intel box. You can run z/OS, z/Linux, z/VM, whatever TPF is called nowadays, and I've never fooled with it but I believe z/VSE can also run native on z/Architecture.
Ed