Source available is specifically if the code is not under an open source license. Whereas software that is under an OS license is open source, whatever the maintainers policy on contributions.

ah, you're right. i missed that.

Not necessarily. There are some true open source projects where the development model is some company "throwing code over the wall". Often they don't make it easy or even possible for anyone outside the company to contribute, they don't have an open repository or bug tracker, release only periodic tarballs, etc. But the license is OSI approved, such as GPL, BSD, etc. so in theory someone could fork the project.

TL;DR - development model != license

> There are some true open source projects where the development model is some company "throwing code over the wall". Often they don't make it easy or even possible for anyone outside the company to contribute[...]

Google's V8 engine is essentially an example of this, though they do notionally have some framework for outside contributions (https://v8.dev/docs/contribute).

Which would be fine for e.g. verifying the results of computations done using Mathematica.

I completely agree that Mathematica needs a steady stream of income to continue, it is a great language and for many use cases the best available. But every time I see a paper that relied on Mathematica for critical conclusions I'm forced to dismiss it as incomplete speculation.