They were probably reaching for the opposite of “is defined by a standard with an open-membership standards body”, found that there isn’t really a word for that

They were probably reaching for the opposite of “is defined by a standard with an open-membership standards body”, found that there isn’t really a word for that, and so used the next-closest thing in concept-space. (Though also, they’re technically correct — “proprietary” just means “controlled by one party.” Open-source things can still be proprietary: namely, if you’re free to fork, but the original upstream can only be merged to by employees of a corporation. I believe e.g. most of Apple’s “open source” is proprietary in this sense.)