No, they are not advertising the ratio. They are advertising reducing both. By your size/weight ratio, the original iPad would be the "ideal", but any one who has used any of these devices knows that's absurd.

Density is far less important than the actual size and weight. High density is a demonstration that they've utilized all the available space (since the density of the components themselves are not changing) for components and need less room for heat. Alternatively, it could be a sign that the internal structure matters less than less and only the exterior components (aluminum and glass) are meaningfully different from one version to another.

What makes the iPad Air attractive is not density, it's that there's less of it all around.