That's pretty vague. How does it apply to this situation?

The code is open; part of the NaCL API is designed around Posix/Linux; the Plugin API is the familiar NPAPI and its extensions.

Which part is proprietary or controlled by Google?

To my mind, some more relevant questions are: Does this tie the web to x86 (and ARM)? Can it be adapted securely to x86-64? Can it be made secure? Can multiple implementations be made secure?