I'm not up on NYC code, since I don't live in the city, but I wonder if it was a rebuild of an existing wooden tank under a "repair in kind" exemption. I have seen shit like that before, such as codes that allow knocking down all but one wall of a building and still calling it a "remodel" (and thus preserving grandfathered code exemptions).
My favorite one is keeping one wall, building three, knocking down the last wall and huraah... still a 'remodel'
Code hacking like that is almost a sport in South Florida.