[cable]

I don't think you can safely presume what I know or think.

There's a substantial difference between how it is used within the local network, for device discovery (ie what Apple's really talking about in that developer article), and how it is used for IPTV, especially where wifi is involved and you can't use that to efficiently transfer large amounts of data in a multicast fashion (it will try to do so at a very low data rate). I will reiterate that BT seemed to have trouble getting it to work with their own set top box, as it required a later firmware update to enable wifi functionality.

I'm not aware of any other IPTV provider using multicast on the Apple TV - indeed Swisscom explicitly mentions that they had to go unicast

www.v-net.tv 

When did I say that I was presuming what you know or think?

device discovery is exactly the point of multicast, that's the point of knowing what devices are needing to receive the packets, i.e. the subscribed group (that's for the purposes of IGMP, not whether or not you are a TV subscriber) of data! its not 1:1 as in unicast or 1:all which is broadcast.

also, Define the local network! at the end of the day your home gateway has a point to point link to BT/EE network. Your typical home gateway uses network address translation to hide your homes internal devices from the wider internet behind a public IP address. if you google what's my IP, it will tell you your public IP, where as if you look on your device it will tell you your local IP (e.g. 192.168.x.x.). effectively the ISPs services which are not exposed to the wider internet are. effectively, for intents and purposes an intranet.