Here's the kicker: the rental units are accessed via the 'Poor Door' with separate elevators. The Condos have another entrance with separate elevators. See, Extell's 40 Riverside to Include Poor Door for Renters
Many things I deal with boil down to semantics. In this instance the developer argues that since the portions are separated, they can be considered to be 'off-site'. Hence the zoning regulations require a separate entrance.
This building model exists already. There are many examples of hotel/condo combinations with separate entrances; there are condo/office models with separate entrances. Right here in Palo Alto, the mostly market rate 800 High Street is, I believe, connected via underground parking to the adjacent but detached above ground below rate 801 Alma.