I had the same issue. I assumed USB-C is not needed, because of how it was expressed.
Appranetly it is needed, and I got it today.
Apart from the fact that it bearly fits together with HDMI cable plugged in at the same time (USB-C has to bend to fit), I still don't see Apple TV on iTunes.
But it appeared in Xcode 7.1 beta 3.
Overall, a bit more clear notice for developers regarding these steps would save some time. Also, because HDMI and USB-C ports are so close to each other, not all cables are safe to use, unless you don't mind risking of damaging device connection port or in a better case - cable.
Also, after playing arround a bit, managed to hang Apple TV. It stopped responding to remote controll interactions, so device stuck in Fly-by mode.
Found that pressing Play/Pause+Home buttons and holding for a bit, does force restart (helpful in my case, didn't need to unplug from electric grid).
Any way, thanks Apple for this Developer Kit 🙂
Now just need crazy and fun ideas.