And to add onto this: pre-ACPI, the IBM PC platform wasn't that far off from, say, any one particular SoC vendor in the ARM ecosystem. In fact, Microsoft originally planned to license out MS-DOS to competing x86 computer vendors using incompatible platforms and firmware[0]. Which is close[1] to the state of non-UEFI/non-PSCI ARM now. It's only because Compaq was able to legally clone the PC and it's BIOS that x86 standardized itself on one platform and firmware.

The presence or lack thereof of standardized interfaces for firmware, device configuration, or the underlying platform are orthogonal to issues regarding Secure Boot and trust management. Apple Silicon Macs use nonstandard boot firmware (iBoot) but booting a "fully untrusted OS" (or fuOS) is an explicitly supported[2] use case on them, gated only by the user needing to boot recoveryOS (OTR specifically) once and enter their password to sign the alternative kernel. They even support per-volume boot policies, so you can keep your macOS install fully locked down while your Asahi Linux does whatever you want.

And likewise Intel isn't stopping you from building in whatever user-hostile nonsense you want into x86 firmware. There's actually a whole range of laptops that have BIOS rootkits preinstalled, specifically to force-install Computrace onto whatever Windows install gets booted for corporate IT management purposes. The thing is, corporate IT has a terrible habit of leaving this shit on laptops they've sold", either because the laptop was stolen internally or because IT couldn't give a shit to do the computer equivalent of signing the title, so people wind up buying laptops that will lock up and wipe themselves if you ever install Windows on them.

[0] The most successful of these being the PC-98, which lasted all the way up until the Windows 9x era

[1] ARM SoC vendors additionally commit the crime of not being compatible with themselves. It is common for new SoCs to have completely different memory and device layouts. Apple is the only exception, ironically because they make both the OS and the SoC, which is the one time where such crimes would be excusable.

[2] I'm told Apple's original intent was Boot Camp with Windows on ARM, but Microsoft wouldn't license Windows on ARM on Macs because they have an exclusivity deal with Qualcomm.