if it is lighter than air, why is it sitting on top of the dandilion and not floating away?
It’s permeated with air. You have to assume that the designation, “lighter than air”, means that the solid material weighs less than the air occupied by the space of the lattice structure, but this space is still mostly filled with air. Hence my remark about forming a “vacuum balloon” held up by this material.
They’re counting the density as the enclosed volume, not as the individual elements making it up. A frame made of thread might be less dense than air (1kg/m^3) but the thread still sinks in air.