> Columnar layout is FUNDAMENTALLY BROKEN on media that doesn't have two fixed-size axes.

You can use plain old CSS columns (which don't have the automated "masonry" packing behavior of this new Grid layout, they just display content sequentially) and scroll them horizontally. But horizontal scrolling is cumbersome with most input devices, so this new "packed" columnar layout is a good way of coping with the awkwardness of vertical scrolled fixed-width lanes.