Interesting. I only glanced at it for now, but the argument seems to be going in favor of modern browsers using CSS grid and older browers either 1) using a well-supported flexbox model or 2) not using anything, but will simply get a mobile-like appearance.
It seems to be that you simply design some containers, then put all the elements in it,for example regular divs - and then you style theses divs into columns and rows. So IE for example, would only see a straight-up design and thus be encouraged to try other browsers..but they would still actually see the content. I like this thought better, because adding a flexbox alternative/fallback means more to maintain overall.