I know you are focusing on the front end of geometry, but with the shift towards historical American designs the rear end of the bikes have changed too. We are see shorter chainstays and steeper seat angles (for a given size) than we did in the 70s/80s. Not only that, we now see a lot of zero set back seatposts, so our weight distribution and how we sit on a bike may also have changed.
Again, not really the point of the article, but don’t neglect that gravel bikes not only have more bb drop and slacker head angles than road bikes, they also have longer chainstays.
Other factors in why Euro brands now have the same geometry as American designs could be: 1) Open Molds; many companies no longer design their own bikes, and use molds that already exist at the actual contracted factory. 2) With the rise of American companies as major sponsors in the Pro Peloton, the competition for high end bike sales has changed. The market is no longer Colnago vs Pinarello per se, it is Colnago vs Specialized and Pinarello vs. Trek. Comparisons are being made, and similarities need to be manufactured.