AppleTV has normally had updates that are in sync with mobile devices. The fact that there haven't been any updates or really any mention outside of peer Airplay playback tells me that there is more coming for AppleTV that isn't ready to be disclosed to the general public.
This basically means for the short term more partnership models for AppleTV updates, but there is obviously a big update coming that isn't really developer ready yet.
I'd imagine by this time next year AppleTV and mystery lifestyle watch health device will be out and expanding things out even further will be the focus for next year.
Pure speculation, but previous iOS's having AppleTV builds and this one not seems to speak volumes to me.
The changes in AppleTV iOS probably has more to do with centralizing TV provider authentication, GUI alignment with the rest of the Core OS family, and enabling peer to peer Airplay. None of which is something that would really directly require open App Store development yet.