As Bob has noted, a multiwire branch circuit is just that 'a' branch circuit as per Art 100. There are other locations in the NEC that permit the use of a multiwire branch circuit as more than one circuit. Knowing these locations and how to understand them is a good benefit to understanding the NEC.
210.4 permits the use of a multiwire branch circuit as more than one - a good place we see this regularly is in kitchens for the small appliance branch circuits.
Pierre
