The term “proprietary” should mean “something that is owned by someone”. Even people who create software (or hardware) that then license under what they consider “open” and therefore non-proprietary licenses are kidding themselves. They own it. They created it. They chose to license it. Even after licensing it, they *probably* still own it (unless they gave up ownership in the license).
Proprietary does not mean secret, non-open, or anything like that … despite that, I agree that’s how the word seems to be now used. I guess it’s ok that words evolve, but the definition of the word “proprietary” has evolved faster than it should given that this evolution has confused the topic of software licensing.