The only interface the computer normally has to the cable is the single control pin on in the middle of the MagSafe connector, so they are likely having to do something more sophisticated than UART.
(Apple could have had you plug the other end that has the USB-C connector into the computer to do the firmware update, but that isn't particularly intuitive or user friendly).
(Apple could have had you plug the other end that has the USB-C connector into the computer to do the firmware update, but that isn't particularly intuitive or user friendly).