For IPTV: My "guess" is that they may have a separate APN for that. It may not be bundled with your normal data usage, otherwise there is no point of low FUP. The revenue model for IPTV could include carriage fees, ad revenues (ads may play before the channel starts), and ofcourse subscription fees from user. RJio may price the IPTV bundle initially for free, and then keep it as an addon for a fixed monthly price - no additional data charges.
This is much better than high pre FUP data, because for mobiles, IPTV will consume less bandwidth than say an HD video on youtube. They will carry low bitrate video for small screens, and that means less data. Also its a best case transfer model (UDP based, if you understand networking), so there could be packet drops occasionally. TV channels may not be as smooth as youtube videos when many people are watching IPTV at once from the same tower, or while moving at a fast speed (in cars, buses, trains, etc).
Hey, I am not discouraging anyone. Its a new and promising technology, people are definitely exited. But if keep the expectations a little lower we may not face disappointment later
Let's see if RJio surprises us!