The rule is only, no macOS on non-Apple hardware.
What on earth are you talking about, iPads, iPhones and all other new devices from Apple outright lockout any kind of 3rd party OS from booting on them.
MacBooks are the only ones grandfathered into old behaviour of actually allowing 3rd party OSes (although they've also lost the ability to boot Windows in latest generation as well).
Apple doesn't really care about individual hackintoshers. Some of their devs have griped about the uselessness of stack traces and logs coming in from hackintoshed machines (bogus errors produced by slight hardware mismatches, drivers developed by amateur community members filling logs with garbage, etc) but they've never gone after anybody who was hackintoshing for personal use. There's even been fairly big YouTubers who've done it without issue, and back when Macs were Intel only and had abysmal thermals/performance a surprising number of Mac/iOS devs were using hackintoshes as their primary dev machines that they submitted to the App Store with.
Where they draw the line is selling hackintoshed machines or any of the tools to facilitate the process. Eliminate financial gain from the equation and you'll probably be fine.
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!"
