These are just NIMBYs latching onto any excuse they can find, but there is a pervasive notion that low density housing is somehow more "green" than urban living.
Aside from health issues from concentrated auto emissions (real but not usually what people mean by "green" these days), this is complete nonsense, especially from a kind of Sierra Clubish "preserve nature" green perspective. What happens if 3 million people move out of New York City and go upstate, for example?
Concentrated urban areas have much lower CO2 emissions per capita for a variety of reasons and more people per square mile of course means fewer square miles eaten up by sprawl.
