I have been booting from external drives on different hardware since 2007. I was even able to trick Windows XP to boot off of a 12GB SanDisk thumb drive. (Although it was horribly slow!)

Coming back to the author's story, as others have pointed out as well, I do not think it is related to the DFU port itself. I think it depends on the BIOS/UEFI firmware which is addressing those ports, and then the bootloader who is responsible for finding the system (root) volume.

Nowadays these happen with Volume UUIDs hence it should not matter, at least in theory. But even GRUB adds a hint, as discovery just with UUID may fail.

Since we cannot see what actually is happening or see the logs, I would simply say: "Always use the same port for booting and installation." Which usually simplifies the process.

I am quite certain "the undocumented DFU port" was the port author initially used to install macOS to the external drive. Maybe on another Mac/machine. When they change the machine, addressing/enumeration of ports may be different, due to how boot process works. Therefore, let's say you used the port=0x3 in the first install, when you change the machine, you need to find the same port=0x3. Thus being the undocumented-DFU-port author mentions.

> P.S: Also DFU port is for installing firmware (BIOS/UEFI) to the device even before boot occurs. For example, you should connect one end of a USB cable to a working computer (ie. "master"), another end to the DFU port of target (ie. "slave") while the machine that is off. Some specific sequence of power-key combination puts target machine into DFU-mode, where you can overwrite the firmware (UEFI/BIOS, etc) from the working machine... That is the purpose of DFU. -- Or at least access the internal hard-drive/SSD without actually booting the "slave" machine.