That's the neat thing. Nobody is. It is a multi-consensus arrangement where you have different parties that accept different definitions.
>in which case you have to somehow demonstrate that's the way people understand it?
Why?
This is a common mistake a lot of people make. "I think X = 1, therefore everyone that is correct thinks X = 1". There could be huge variation in subgroup understanding on what this should be, and as long as there is no overarching legal requirement then you could consider them all correct on matters of opinion.
It's annoying that people pretend like Open Source was a common phrase before it was invented to subvert Free Software by OSI, and it's shameful that people would tell this lie for the sole purpose of trading on the goodwill that Open Source software has created. Even in the world where people were always talking about "open source" before the OSI, that talk is certainly not the reason that people who are using the term to describe non-OSI approved licenses are using it today. They're intentionally trying to defraud people, and have the nerve to be snotty about it.