However, just because Apple "runs linux" doesn't mean they "run linux" the way you are thinking. It's very easy for corporations to write slapdash, horrific, unmaintainable kernel forks that run on a specific piece of hardware. That's just fine when you are testing hardware before handing it to your OS team, but absolutely unacceptable for upstreaming.

For examples of this, take a look at old Android devices (and their ancient kernels), or the original Correlium port of Linux to Apple Silicon (which happened almost half a year before the Asahi Linux beta - but the code was sheer unmaintainable crap). Upstream it? Heck no - it would be rejected entirely and need almost a total rewrite from scratch. Just because you can write a functional driver doesn't mean it is anywhere close to a good, maintainable driver.

So, in a nutshell... yes, Apple does use Linux for early manufacturing tests. But it would almost certainly not be in a state where we could benefit much from it, and certain features would likely not be implemented. It's not anywhere near as simple as "Apple has done the work already - just upstream it please!"