No, it is just the bootloader (same way the Android phones work), though MS could craft their signed bootloader to also verify the NT kernel (which would make sense).

The other problem is, in the original article that was published on this topic, that apparently the Linux/grub boot process will be changing so that the "kernel is part of the bootloader", so I think that adds to the complexity of the idea of signing either the bootloader or "the whole OS" (whatever that means anyway.