mod- well, no
uAvionix has invented a way to read the transponder code and altitude data from the DC power bus. Very clever. Garmin does actually "sniff" the RF signal. They sued uAvionix but lost.
having said all that... I have installed both ways. The FAA doesn't care... just need a good report after a test flight.
The mux is a serial combiner. They will send you one for free. It allows connection to the Garmin transponder which is usually not needed for other transponder brands that have more common serial connections.
Now, after seeing the method used by uAvionix, I still wired to my Sandia transponder and I feel more confident of a 100% signal transfer at all times using RS232 data wire.
I park near a ground station and have had good PAPR reports with as little as a 12 minute flight around the pattern. Most builders will have to use the more common guidance for a test flight.
Best of luck and call uAvionix if confused. They have tech support that is well regarded in the industry.