I think it's not only about their proprietary source code, but also about restricting code that can run on your computer. Newer Macs have trusted boot, where the T1 / T2 coprocessor makes sure that system software has not been tampered with before booting. And every release seems to go further into the direction of verifying the code you execute.
As far as I understand it, if the GPLv3 clauses would apply, Apple would have to allow anyone to run their own, modified version of macOS on their computer, meaning Apple would need to provide a way for people to circumvent / disable all the secure boot architecture.
Also, things like "Activation Lock", where Apple requires your Apple ID password to re-install the OS (a theft protection feature) doesn't sound like it would be easy to reconcile with GPLv3.
I think they already allow modifications? Apple lets you disable System Integrity Protection [0], after which point you can make any modifications to the system. Only the T2 supports Secure Boot and it can also be disabled [1].
"Activation Lock" is an iOS thing. On Macs you can set a firmware password [2], but I don't see how that would conflict with GPLv3.
If a judge and jury find that the T2 chip, macOS, and bash4 were all integrated components of the software package “macOS” for the purposes of GPLv3 copyleft provisions, then Apple would be forced to share the source code of all software running on the T2 chip as well as all software it bundles with macOS.
While we can armchair legal this in either direction on how likely that judgement would be, they appear instead to have decided to never permit that risk to occur at all. I approve, too: if you don’t want to share every bit of source code in your product, don’t use GPLv3 in your product. It’s explicitly meant to be annoying like that, and any plausible effort to have your cake and eat it too risks being overturned by a court of law.
As far as I understand it, if the GPLv3 clauses would apply, Apple would have to allow anyone to run their own, modified version of macOS on their computer, meaning Apple would need to provide a way for people to circumvent / disable all the secure boot architecture.
Also, things like "Activation Lock", where Apple requires your Apple ID password to re-install the OS (a theft protection feature) doesn't sound like it would be easy to reconcile with GPLv3.