You're arguing about linguistic prescriptivism vs. descriptivism. Is the definition of the term what the coiner says, or is it how it is commonly understood?
A lot of people are going to disagree with you if you claim that prescriptivism is somehow more correct.
Perhaps, but "open source" isn't a technical term, and anyway there are lots of vaguely technical terms that have a subtly different layman's definition anyway.
Categorizing certain phrases as "technical terms" to which different rules apply is just another form of linguistic prescriptivism, after all.
"Open source" is very much a technical term with a very precise definition.
Microsoft knew this when they wanted to jump on the bandwagon and named their own license "Shared Source Initiative" (https://en.wikipedia.org/wiki/Shared_Source_Initiative). It wasn't open source, it was "Shared Source", because it didn't comply with the definition of open source.
"Open Source" specifically was coined precisely because there was little or no documented previous use of the term. They wanted something that would mean roughly the same as "Free Software" but possible to protect with a trademark and owned by an industry association. (Not the FSF.)
Related, the definition of gif has actually changed for some. Instead of a specific file format they use the term to mean "soundless looping video". Admittedly even I have a hard time swallowing that definition.
A lot of people are going to disagree with you if you claim that prescriptivism is somehow more correct.