This doesnt seem to gibe with a breed developed that long ago on a farm.
The thing we have to remember is, in the early part of the 20th century, farmers fed their birds "meat scrap", which was often all the offal from the slaughter of cows, pigs, sheep, or other large animals. Little was wasted, and chickens are aggressive omnivores who will eat just about anything. Buckeyes in particular are good hunters and will actively hunt mice, snakes, lizards, bugs, and anything else that moves!
So the breed doing better with higher protein levels, despite being developed on a farm actually makes sense, as it no doubt had access to the offal from slaughter. I don't know for sure what other livestock Mrs. Metcalf raised, but if it was a working farm typical of the day, you can be sure there were more animals than just chickens.
I am told (although I haven't done it yet) that one can go to a small grocery store where they have their own butcher, and ask for a bucket of "meat sand" (I think that's the term), which is the small scraps left over from the butchering process. This can then be fed to the chickens, who love it! Certainly worth a try if you're seeking a cheap source of animal protein.