Multicast only works on either something as esoteric as the MBONE or on a local LAN/WAN. It isn't out there on the general internet, it requires too much state in the routers, so it never took off.
The original comment was not on transport level multicast, but on application level multicast. This is the one implement in ejabberd patches and thus works over Internet, not only LAN/WAN.
Transport-level mcast requires special routers..
Ah! No wonder I couldn't get the MBone videoconferencing stuff to work with Trumpet Winsock, years ago