Except.. it kinda is. I mean yes, it is obviously true that words can (and do) have multiple meanings. But within a given context, there are "generally accepted" definitions and there are idiosyncratic / obscure / unusual definitions. And in the kind of context we're talking about here, to use "Open Source' to mean anything other than what is defined by the OSD is to intentionally choose a non-standard, idiosyncratic definition. It wouldn't be reasonable for one to do that and then expect everybody else to understand / agree with their non-standard usage.
The way most people have been using “open source” is a lot closer to OSI than these companies are claiming. It’s misleading to use a different definition nowadays. And I think that’s the actual intent. If it wasn’t, then they’d be perfectly fine with using another unambiguous term like “source available.” If there was a strong desire to be clear and not misleading, they’d use such a term.