The reversal was based on the Environmental Control Board's holding that the New York law at issue, which states that residents cannot rent out all or part of their property for less than 30 days, allows people to do so if a resident of the space is in the unit at the time (in this case, the Airbnb user's roommate was in the apartment).
Whether it is legal to rent out a New York City dwelling for less than 30 days when none of that dwelling's regular residents is at home is still unclear.