That's what TFA says. I imagine that the latitude matters, too. Near the poles, the sun is coming in at such an oblique angle that any given surface area isn't

That's what TFA says. I imagine that the latitude matters, too. Near the poles, the sun is coming in at such an oblique angle that any given surface area isn't getting much heating effect from it anyway, so it's easier for the insulating blanket effect to be a net positive.