https://support.apple.com/en-us/108900
That said, no idea why they can’t be used in this case
Also on my mbpro at least the mentioned port is the one closest to the magsafe connector and may have funny electrical connections to it, perhaps.
My intuitive guess here is how the ports are connected to the T2 security chip. One port is as you said a console port that allows access to perform commands to flash/recover/re-provision the T2 chip. Same as an OOB serial port on networking equip.
The rest of the ports the T2 chip has read/write access to devices connected to it. Since this is an OS drive, I'm guessing it needs to be encrypted and the T2 chip handles this function.
The rest of the code necessary to boot from external sources is located on main flash
This is like saying my software did not work because it was based on an incompatible version of some library. Maybe so, but that is a bad excuse. Implementing systems is hard, and like the rest of us, Apple should not get away with bad excuses. And this is even more true because they control more of the stack.
The 3 users of this feature on this planet are already happy that it’s even possible at all. The only thing Apple could do is to document this clearly like adding a text in the boot drive selector.