Who says it isn't? "closed source" doesn't have a formal definition, but can be arbitrarily defined as the antonym of open source, and when people use the term

Who says it isn't? "closed source" doesn't have a formal definition, but can be arbitrarily defined as the antonym of open source, and when people use the term that's usually what they mean. And that has nothing to do with whether someone can be "blamed" for ignoring the actual meaning of a term with a formal definition.