Thermal insulation, whether we're talking about a blanket, a winter jacket, or the foam stuff in your walls, generally involves a layer of some thickness within which there are many tiny spaces full of trapped air. It's difficult for heat to propagate across those spaces. The author's point is that mylar emergency blankets do not provide this thick layer of "trapped air". It's true that they prevent air from crossing, but only for the very thin layer of the blanket.

I'm not sure exactly how effective the mylar blankets are and under what circumstances, but this is what "trapped air" refers to.