Real usage by real people not particularly passionate about adherence to the OSI definition—to me this is its de facto meaning.I'm not talking about "people who are particularly passionate about adherence to the OSI definition" though. I'm talking about people who are "particularly interested in the actual technicalities of what OSS is", not all of whom may agree with the OSD. But I still argue that such a significant majority do that it constitutes the de facto definition.
Your response assumed the number of people who use the phrase with a looser meaning is small;
Not at all. I am saying that the people using that phrase in the "looser" sense, as you put it, are using it in a colloquial and not technical sense, and that such usage has no meaning as far as what the de facto meaning is, when used in an actual technical context. That's just lack of knowledge, not any attempt to create a different definition.
I see it more like somebody who doesn't know much about cars referring to an engine block as a carburetor. Even if a lot of people make that same mistake, it's still a mistake and the actual definitions of "engine block" and "carburetor" don't change.