Some swear he speaks 100% truth. Just as many swear he is 100% full of crap
Like everyone else, Elmer and Jack had their opinions and were not shy about talking about them. Does that mean they were 100% right all the time? No, But when you mix a bunch of experience with a spirit of adventure you get results. Not always what you wnat, but you get results. For example, Elmer Keith wrote in one of his books that the .25-35 was a killing machine and the best elk rifle ever. Of course, he said this after relating a story about him and his brother shooting an elk with a 25-35 (because that's what they had) then chasing it for 2 days, shooting it a number of times, until finally it collapsed. Probably not the results Elmer wanted, but they ate elk that winter. The only people who don't get results are the ones who never try.
Elmer Keith was born in 1899, Jack O'Connor in 1902. Smokeless powder was just starting to be in common use when those 2 were still in diapers. It took time for people to figure out just how much of a difference smokeless powder made over black powder.
When smokeless powder was introduced, it obsoleted a whole trainload of cartridges virtually overnight. The 30-30 Winchester and the 30-40 Krag were already in wide use by the time those two were born, and in the 15-20 years following (before they were grown) there were some really awesome rifle cartridges introduced (30-06, 300 Newton, 256 Newton, 250 Savage, 33 Winchester, 348 Winchester, 405 Winchester, etc, etc). Elmer liked to hunt big African animals, and large bullets work best when hunting animals that big. Jack O Conner, OTOH, liked to hunt deer, sheep, antelope, elk, mostly North American big game. Elephants, rhinos, and hippos are hunted close up in heavy brush where you might not get a second shot if you miss. Antelope and sheep are hunted in the wide open and long shots are the norm. Both of these men advocated guns that worked for the way they liked to hunt and what they knew about. Elmer Keith also had the advantage of having worked in researching and developing cartridges while in the Army, while Jack O Conner learned a lot from books and was paid to write by Winchester and promote their products. Both were opinionated and hard to convince of anything other than their own opinions.
I read a lot of Jack and Elmer's stuff (along with dozens of other writers) while growing up and never got the impression that either of them was blowing smoke, they were simply telling a story. I read a lot of other writer's stuff as well because I wouldn't let an outdoor rag get by me without reading it cover to cover. I laughed at the squabbles the different writers would have over unsubstantial issues (220 Swift vs 22-250, .222 vs 223 or .270 Winchester vs 280 Remington, for example), and at other times I would recite the stuff I had read as if it were pure Gospel. So read and believe whatever you want, but take it all with a grain of salt.