I don't think I agree with the author. While of course a cohesive and disciplined army will do very well against a band of loosely affiliated warriors, the timescales of natural vs social evolution are just wildly different. A society that has spent 30 generations breeding strong warriors and 3 generations forming disciplined armies will likely decimate the society that spent 30 generations forming disciplined armies and 3 breeding strong warriors. But more than just physical fitness, harsh conditions tend to select for adaptability and ingenuity whereas more forgiving conditions favor stability and orthodoxy.
We see this pattern played out throughout history. Generally big agricultural societies form in good fertile spots which place strong emphasis on social cohesion and discipline. These cultures form large and well organized militaries which regularly fight other armies from the same region and develop an extensive military tradition. However these societies lose terribly to, and wind up being ruled over by, much smaller populations from the inhospitable periphery such as hill people or steppe nomads. Consider the Hyksos invasion of Egypt, the Mongol conquest of China, the countless wars over Mesopotamia. Even for the cited case of the Romans, they started out living in swampland and hills on the periphery of Etruscan civilization, and despite centuries upon centuries of military tradition they would fall to various periphery groups like the goths, huns, bulgars, etc. Of course every empire needs to fall at some point and its conquerors need to come from somewhere, but it's still notable that the peripheral societies accomplish this at such a numerical and resource disadvantage.
I think in real life any society that developed stillsuits or worm riding and mastered survival on Arrakis would be unparalleled when it came to traversing hostile terrain, and logistics would come as naturally to them as breathing. Sure a more traditional force might be able to best them in a pitched battle, but they'd bleed any force white long before such a battle was fought.
The historian bret devereaux has a multi-part and thorough evaluation of this trope through history, which he calls "the fremen mirage" so clearly based on the same dune example of it. He also strongly agrees with the OP's suspicion: it's a mix of myth, propaganda, and storytelling device depending on the exact context it's deployed, but completely unsupported by history. It's a good read.
We see this pattern played out throughout history. Generally big agricultural societies form in good fertile spots which place strong emphasis on social cohesion and discipline. These cultures form large and well organized militaries which regularly fight other armies from the same region and develop an extensive military tradition. However these societies lose terribly to, and wind up being ruled over by, much smaller populations from the inhospitable periphery such as hill people or steppe nomads. Consider the Hyksos invasion of Egypt, the Mongol conquest of China, the countless wars over Mesopotamia. Even for the cited case of the Romans, they started out living in swampland and hills on the periphery of Etruscan civilization, and despite centuries upon centuries of military tradition they would fall to various periphery groups like the goths, huns, bulgars, etc. Of course every empire needs to fall at some point and its conquerors need to come from somewhere, but it's still notable that the peripheral societies accomplish this at such a numerical and resource disadvantage.
I think in real life any society that developed stillsuits or worm riding and mastered survival on Arrakis would be unparalleled when it came to traversing hostile terrain, and logistics would come as naturally to them as breathing. Sure a more traditional force might be able to best them in a pitched battle, but they'd bleed any force white long before such a battle was fought.