While I'm sure your assessment of American workers is correct, your point about Dr. Sowell "miss[ing] the reality" doesn't follow.
Sowell's point is that the market will determine the availability of labor, whether competition is from immigrant labor or automation. For (even) the Wall Street Journal to fall into the traps he has written about recently, i.e., the "abstract" immigrant and the "need" for farm labor, is disconcerting. The thinking by WSJ columnists and editors is usually logical and supportive of good policy. This just illustrates again the subtlety and political baggage that clouds issues about public policy.
The important point (I think) is that we should have clear eyes about the impact of immigration, realize that all immigrants are not the same, and let the market determine how we allocate resources to meet "needs," whether they be for labor, food or anything else.
This approach, which humans are so reluctant to employ (because it appears to relinquish necessary "control"), is the way to maximize our economic well-being.