That's an odd choice of phrase; it's unfortunate that a press release chooses to disparage alternatives without explanation.
Here are some definitions of "proprietary" as used by members of the FOSS community when talking about standards:
>"Proprietary" as in "sole proprietor" is appropriate for a project with zero governance, launched by Google after some incubation closed-source, dominated by Googlers.
https://news.ycombinator.com/item?id=9395992 (pcwalton, Mozilla employee and Rust core developer)
>In a competitive multi-vendor ecosystem like the Web, public-facing protocols that are introduced and controlled by a single vendor are proprietary, regardless of whether you can look at the source code. NaCl and Pepper, for example, are proprietary, even though they have open-source implementations.
Yet when applied to Google's products, this is suddenly viewed in a different manner? Even if the maintainer does not accept patches, you can still fork it, so no freedom is lost. And it's ok for other people to make a proprietary fork, but not ok for the author to make a proprietary fork? That sounds like hypocrisy to me.
Forking standards is completely different to forking a codebase. It should be obvious why.