> [EDIT:] I looked around, but unfortunately I don't appear to have the proper cable to perform a DFU test. In fact I usually need to use some damn dongle just to connect to USB-C.

FYI, USB-3.0 C-to-A dongle + USB-3.0 A-to-C cable definitely works (haven't tested USB 2.0). The C side of the cable needs to be plugged in to the machine being DFU'd not the host machine where the dongle goes.