There are very few if not zero modern computers with completely open source firmware and boot loader. Including computers with open source as their primary selling point.
Some people find it psychologically easier to sit in the shit when they discover they are not alone. Some even go further and trying to justify the shit when there is a lot of it.
I am not one of them, so for me shit is still shit and I prefer to see it for what it is.
I am not suggesting it is perfect, but that it may be a little hyperbolic to say any os you load on a Mac is not it's 'own OS' because it relies on closed bootloaders/firmware. If that is the case, then what does every other computer run? Is Linus's own operating system not his own because he loads it on an Intel or AMD processor?
We will know the answer to that question once we study thoroughly all versions of microcode those processors have/ had. Alto had how much? 128k ? and some big part of it was for display memory? And it was a full OS. Just imagine what you can have in firmware/bootloader now days. Is it that hard to imagine for everybody?
Also, from the same source: “ Will this make Apple Silicon Macs a fully open platform? No, Apple still controls the boot process ...”
Without open firmware and bootloader ... well, is it really Own OS?