I mean the dictionary definition of "proprietary": controlled by a single proprietor, in this case Google. Google proposed the Pepper API on the plugin-futures mailing list a year or two ago, but Mozilla and Apple believed that it was unnecessary to maintain a parallel Pepper API for plugins to use when Web APIs were already there (and could be improved where they were not sufficient). Google implemented and pushed Pepper anyway. Thus, proprietary.
Likewise, Google is not open to input regarding the direction of NaCl, or whether it is a good idea for the Web. It is also proprietary.
Whether proprietary-but-open-source technologies are good for the Web is a separate discussion -- clearly, Mozilla and Google disagree. But it's unarguable that NaCl and Pepper are proprietary technologies.
Likewise, Google is not open to input regarding the direction of NaCl, or whether it is a good idea for the Web. It is also proprietary.
Whether proprietary-but-open-source technologies are good for the Web is a separate discussion -- clearly, Mozilla and Google disagree. But it's unarguable that NaCl and Pepper are proprietary technologies.