Beyond DC makes the point that “density” and “tall buildings” are not synonymous. And, indeed, they’re not. You have parts of the country that basically consist of tallish buildings surrounded by large plazas and parking facilities that are all separated from each other by very wide roads. That kind of tall buildings is not a path to walkable urbanism or environmental sustainability. All very true.
That said, in the specific context of the Washington DC central business district, building denser and building taller largely are synonymous. If you walk around, essentially every building has the largest footprint it’s allowed to have, and essentially every building is built-up to the (very low) maximum height permitted. Downtown Washington has by far the most transit connectivity of any place in the metropolitan area, and so putting more stuff there as opposed to someplace else would make the region more environmentally friendly. But the only way to fit more stuff into Downtown Washington would be to either pave over the National Mall (bad idea) or else build some taller buildings.