It is not a generic term for source-available software, and never was.

The problem with that reasoning is that precedence and origin do not necessarily define language use after it. Language evolves. Society and communities make up new or change definitions.

Misuse of the term is evidence that it's not universally understood to be one way.

I think it's mainly because "open source" can be understood as accessible, readable source. And many people seem to intuitively understand it as such. The "free" terminology on the other hand has a more ambiguous meaning between freedom and no cost. And early on, the "freeware" terminology was established as a differentiation to "free software". "Open source" does not have such an equivalent established differentiation (like "source-available", which seems to be just not as prevalent, maybe because there have been much fewer products with that alone).

I understand the desire to correct, specifically with the established OSD. But I have to wonder if it will ever bear fruit, given these circumstances. And in consequence, whether it's even worth to point out.