Just think how insanely unlikely it would be for this not to be the case.
One could also increase the density by allowing non square shapes or squares not fixed on the grid and so on. A more general approach would always lead to slightly higher scores.
Perhaps something akin to a Hilbert curve that snakes around the whole country encompassing every individual person’s footprints but otherwise covers almost-zero area.
It should be theoretically possible to re-develop it for other spaces (it's "just" a hexagon tiling), but the H3 team put a lot of work into the details and you'd be taking on a big effort redoing that for a different space.