It's great that the phrase "open source" has been so intuitively understood but to claim it has an obvious literal meaning is just nonsense.
An open door implies you can see what's behind the door, as opposed to a closed door. An open book implies you can see what's inside a book, as opposed to a closed book. Likewise, open source implies that you can see inside the source code, as opposed to closed source.
So I think the claim that it has no obvious literal meaning is a bit hyperbolic, but I get where your thinking, since most developers automatically associate "open source" with "free code".
Perhaps Glass Source would be a more accurate phrase for code you can see but not touch.
No that is how the English language works. Car gas and gas are two different things.
Source available is akin to putting a paper behind a piece of glass, how is that open? You can't touch it, can't modify it.