The microclimate conditions of a roof in your area bear similarities to alpine zones above treeline in Vermont, Maine, and New Hampshire where drainage, soil type, wind, and lack of shade dictate very different plant mix from the surounding valleys. The biodiversity is definitely lower that a canopy forest, but not insignificant. And even if this habitat is typically found a few hundred miles to the North, it's still part of the regional range of habitats through which many local species migrate.
Someone who knows more about species diversity and habitat quality would have more to say on whether your plant mix is comparable or not, but it seems any "constructed green space" on a roof top is going to face microclimatic challenges that would require a very different mix from what can be planted at grade.
It might be worth a closer look to see if there would be mix of plant species that's appropriate for an extensive green roof that could still provide a meaningful amount of biodiversity and habitat. The reference guide for this credit emphasizes the importance of eco-systems services, stormwater management, even including "exposed rock and bare ground" since even those play a role in habitat diversity. The credit intent also seeks to minimize fertilizer use, irrigation, pesticides, and maintenance costs, which many extensive eco-roof plantings can do.
Sure, we can't restore enough habitat on a green roof to provide for the occaisonal Moose, but perhaps there's a way to determine what's a reasonable amount of plant diversity for roof-top "alpine zone" habitat restoration.