I think that's understandable, it's primarily a security chip so if you're ever going to lock anything down, that would be it. OK, from a purist perspective that's arguable, but I think it's also arguable allowing unsigned code would make it less secure. Anyway you can still run another OS on a Mac with a T1.

Also not true. Someone just needed to write Linux drivers for that hardware.

The thing that initially kept Linux from installing on those Macs was the lack of a driver for the SSD controller which is located on that T1/T2 chip.

https://t2linux.org/

Developing a SSD driver does not require unlocking the device, though.

We're talking about the ability to install Linux on the hardware.

The drivers to do so exist now.