There are “states,” in this Union, where AR15-like rifles are banned. There are entire countries where one cannot hope to own any autoloader, or, where ownership is possible, but one cannot use auto-loaders in a hunting context.
The forward-mounted, intermediate-eye-relief, fixed-power scope is no longer such an advantage, as variables have become more durable, and, the brightness of normally-located scopes has gotten better.
Both books reflected forward thinking, each in its time. Let us keep in mind that Col. Cooper was promoting a low squatting position, which he called the “rice paddy” something, when most pundits were preaching only prone, sitting, kneeling, and the standing positions. (Sorry, most of my books that survived the last move, and then Hurricane Harvey, are in storage, so not available to confirm terminology.)
I would say that neither author is irrelevant. The AR user can benefit from some of the principles and philosophy in _Art of the Rifle_, and the turn-bolt folks should study Kyle Lamb.
Edited to add: I reckon that I fall more into the category of folks who appreciate the “lightweight practical rifle” more that the original “scout rifle” concept. The only turn-bolt rifle I presently own is a Winchester Safari Express, so I have not yet fully “arrived,” in either category.