Even though OSI clearly defines what "open source" means, it is sometimes (often even?) used as a synonym for "source available", as opposed to "free software" (which is the term that FSF promotes).
I'm not saying which term is better, just explaining why "open source" might not be objectively wrong in this case.
I'm not saying which term is better, just explaining why "open source" might not be objectively wrong in this case.