Nobody is arguing that unskilled laborers are incapable. We are saying that they are by definition unskilled. These skills are slow to teach and learn. Having lots of people willing to perform skilled jobs is not enough - they need the skills. The same is not true about unskilled work. Hence the comment about armies of people being able to perform unskilled labor.
I just like physical labor. I could be in some open floor plan trying to write code in some langulag designed by an unsophisticated person over the weekend while interupted by slack nonsense and by a manager who cant tie his own shoes. One coworker is an accountant and annother is an industrial designer. Im incredibly fit and my brain is reserved for private use only.
The dichotomy doesn't have to be perfect to be useful. There are certainly people who are incapable of anything else - and the US military has also pretty consistent in their statements that there are many people who are incapable of anything useful (in the military).