If it's about farmer walks, or ‘walking the load’, then the author is on point.
Farmer walks can be adjusted in different ways to train pretty much the entire body and different systems of the body: cardiovascular, nervous, or muscular system.
up stairs to prioritize the legs
one-sided only to build strong obliques and back muscles
odd handles to build crushing or pinching power
running to build power
super-heavy to build shoulder and arm muscles
light-weight for long walks to burn calories and maintain muscle while getting older
And these are a few examples, the farmer carry can be used in many more ways for more specific purposes, it is safe to assume that yes one can build fitness from just walking with weights.
Literally walking your way to a healthier life or and stronger body.