If you had Linux on a MDM-enrolled Mac there wouldn't be anything MDM-related running during or after the boot process. But presumably any sane MDM config would prevent the end user from accessing the settings necessary to relax boot security to get Linux installed in the first place.
But, eh, I still think it's fair to describe it as a feature of the firmware. The enrollment and prevention of removal have firmware-level components through Apple's Secure Boot and System Integrity Protection. A user can't simply disable MDM because these firmware-level protections prevent tampering with the enrollment.
Case in point, getting Linux installed in the first place would be blocked by firmware-level boot policies, right? I'm not too knowledge about this, and maybe you are more so.
It's not too different from scaremongering about Intel ME/AMT which is often maligned even in the context of computers that don't have the necessary Intel NICs for the remote management features.
But it's still entirely factual in my own description. When a device checks in during initial setup, the firmware-level boot process can receive policies that block alternative OS installation, and that absolutely is a feature of the firmware.
Anyway, I tried to interpret OP's meaning, and provided more detail on how Apple's firmware is special.