You can query for aspect ratio in CSS, yes. This can be useful for figuring out whether it's better to place something as, say, a sidebar vs a footer. The sites they are complaining about, however, are only looking at the horizontal dimension.

I think primarily that's for constraining a div's aspect-ratio. Like making sure you have a div that's always 16:9 to hold a video player. I'd be interested in hearing other use cases, though.