The Mac hardware and macOS "do what you want knobs" are actually pretty open as far as computer hardware goes. Hell, Apple even made part of the process of booting custom kernels easier for the Asahi team after the posted about it. I mean it's not a Talos workstation with full open firmware blobs but it's also not preventing you from loading whatever you want on it... but that could be said of anything.

iOS based devices are where the lockdown is.