Hector Martin claims so which (frankly) suggests he has some insiders.
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!"
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!"