This topic interests me too.
I was once under the impression that IPTV was a standard, and that it used UDP protocol over IP, rather than TCP/IP. That was an important distinction, since UDP does not retransmit lost/corrupt data. In turn, that meant (iirc) that IPTV over DSL might mean operating with a higher SNRM, to compensate for lack of Re-tx.
I now suspect that I was wrong, and that IPTV is (or has become) a fairly vague term used to define any TV that is delivered to the end user over any IP protocol. This simply distinguishes it from terrestrial or satellite broadcast, among other delivery methods.
Even when watching “live” freeview, my own TV is fed from a Myth TV server in a different room, with an ethernet connection to the front-end, under the TV. So even though it is live freeview, is that IPTV, by accepted definition, and if not why not?
Happy to entertain other thoughts, I’m no expert!