Yes it comes close to the open source branding, no it is not the open source branding, they even distance themselves quite clearly right in the header of the page.
What does the word "open" have to do with extensibility? Absolutely nothing. You had to phrase this as "open for modification" to make your point, why isn't "open" alone sufficient? My local museum is open from 10 AM to 8 PM daily, at least when COVID-19 is behind us. Does that mean I can bring my own exhibits, or remove the existing ones? No.
"Open" alone is fine. "Open source" has a specific meaning. "Free and open" is deliberately cribbing off of the well-known phrase "free and open source". How about this: why is "open and free" less suitable?
"Open and free" sounds like a great suggestion, maybe we could lead with that? Maybe I missed some discussion about that suggestion, I only responded to your remark about the word 'open'.
Or are you saying that because I'm stating that open is referring to the licensing, that implies the free refers to the gratis part? I guess that's correct, but that's not a point I raised earlier so I'm not inconsistent in my position.
No. That didn't begin with me. This entire discussion is filled with people defending your use of "free" to mean gratis.
> I'm pointing out that that's not what we're talking about
Okay, well now it's not clear what you're trying to communicate with the word "free".
My point was that if you were using the word "free" to mean gratis, then the best thing would be to call it "royalty-free" instead. That's already the accepted nomenclature for creative/industry use, anyway, and it'd be the least confusing choice for this case in particular.