Yep, if you're plugging a USB stick into one of those USB-C ports and you're seeing the MP3 files then its carrying data - thats very positive news. Out of curiosity, on your car with V11 - were you missing Sonic and Cuphead from the Arcade section when you collected the car? If so, do they now appear with the USB ports installed?
EDIT: Interestingly, the parts for the retrofit on the service estimate for my appointment later this month are different to those you got put onto your car.., hmm...
I wonder if there's an escalation/someone I can email at Tesla who can clarify this stuff. I really want that USB data functionality.