In Jeff Cooper's mind, the purpose of a scout rifle was to carry a handy size and weight bolt gun, in a caliber powerful and flat shooting enough to reach out there and deliver a hurting blow, with desirable shooting characteristics like an easy to run bolt, appropriate shape and length stock, good trigger, shooting sling, and reasonable capacity. This rifle would allow you to boondock in the backcountry, take any game in North America, and defend yourself as individual rifleman in big country (as opposed to a many person, close urban combat scenario).
At the time he envisioned and developed the scout concept, we did not have well constructed .223 bullets, ergonomic AR carbines with free float tubes and good triggers, calibers other than .223/5.56 in an AR, or quality low power variable scopes. Jeff would never have considered an AR or .223 appropriate as a scout rifle, but times and the technology have changed, and I bet he would warm up to something like my lightweight Wilson .308 AR.