Arguably it is, in the sense that they didn't actually invent the term; there are many documented pre-OSI uses (including by high-profile folks like Bill Joy) saying "open source" to just mean "source available". And OSI's attempt to trademark the term was rejected.

> if you don’t welcome outside contributions, it isn’t open source

That isn't even part of the OSI's definition, so what are you basing this on?