Marx built his infamous labor theory of value on the premise that labor itself was a commodity. However, as Mises and other Austrians have noted, Marx failed
Between the fact there there isn't any quoting or additional context of anything Karl Marx wrote, and that the Mises Institute and its founder Lee Rockwell are historically supportive of the Confederacy, I would argue this source isn't a good representation of legitimate critiques of socialist economics.
So this article has one good point: labor should not be thought of as a commodity. But the rest of the article is complete nonsense. The plumber in question owns his means of production which this article totally skips over.
So his own labor is not a commodity then, or is it? You think the author does not know the plumber owns his own labor? That is the point of bringing up the plumber as an example. If labor is an undervalued commodity, then the plumber is selling his labor at a loss in a capitalist system. I understand the point quite well. Apparently, some people don't get it, and it is not a difficult thing to understand. If the self-employed plumber is losing money due to his own labor, then he isn't making a profit. Simple example of the contradiction.
No, labor is not a commodity. I do see that the article now points out that the plumber owns his means of production. The question of whether or not the plumber is undercharging for his labor uas nothing to do with the question "is labor a commodity". That plumber knows what he is doing, and cannot easily be replaced by another individual unless the replacement individual has a similar skill set. The plumber's labor differs from the labor of his replacement. Labor is a specialty, not a commodity.