I don't see how they can guarantee that unless they can maintain completely separate backhaul paths from wherever the linear channel bandwidth is measured and controlled (cabinet/exchange/wherever). To take it to an extreme, if all or most Infinity users connected to a cabinet or exchange were to be watching one HD linear channel and recording another at the same time (hypothetically the World Cup final/Champions League final etc.) there would be an obvious risk that they'd have to take a big chunk of bandwidth from all ordinary Internet users in the area, to maintain linear channel quality, and that it would make a big dent in everyone's Internet speeds at the time.
I'm sure they will plan for this possibility and I just hope they get it right.
Why D@ve, you already know about Multicast. One network signal. Many users.