I can’t imagine the downvoting being anything other than a disagreement with that word usage. Strictly speaking words do not have definitions which are “true” in some innate sense - they have usages which are popular or unpopular among different groups of people.

The term “open source” without any context describes “source” being “open” - as clear as mud. With context that describes a range of software licenses and the disagreement is in which licenses that includes.