That isn't true. Open source refers to the ability to make use of the source code if you wish, not the ability to send pull requests. SQLite is open source (public domain even!), but does not accept contributions from outside.

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?

It's not a question of belief. Maybe words don't mean anything anymore, but certainly legal contracts and licenses do. "Open Source" is a class of licenses approved by the OSI. There are no spirits involved.