The report looks at residential capacity in particular, and find that in many of the rezonings did not significantly change the buildable capacity of the area.
From the Furmen Center's press release: "Given the scale of rezoning activity during this time, it is critical to take a step back and ask: 'what is the net impact on the City's capacity to accommodate new growth?'" said Vicki Been, faculty director of the Furman Center. "While we find that on paper, the upzonings have added more capacity than the downzonings have taken away, we also find reason to doubt that all of this new capacity will be built out for residential use, and it remains unclear whether we are on track for creating enough new residential capacity to accommodate the one million new New Yorkers that are expected to live in the City by 2030."
The planning department responds in the NYT: "Amanda M. Burden, the city's planning director since 2002, said the city's approach to zoning was based on a "finely grained" process of listening to the needs of separate communities and neighborhoods. "We respond to communities where the threat is the greatest to the neighborhood fabric," she said. "We upzone where it's sustainable, and where reinvestment is needed."