The author complains "For some damn reason, it matters which port your external disk is plugged into when you install or update macOS".

The reason is simple and perfectly understandable.

DFU is very low level. It happens very early in during Boot ROM and before the Mac has even entered Low-Level Bootloader. Which is why its also USB-C with no Thunderbolt support.

Boot ROM code is, by necessity and for robust security, kept to a bare minimum.

Bus 0 Receptacle 1 is designated the DFU port in the Boot ROM.

Hence the limitation to one port.

Widening support to >1 port would mean you would have to introduce extra logic into the Boot ROM code (port iteration, conflict resolution etc.).

Hence the K.I.S.S. principle wins. One port.