... all worked as hard as their health & ambitions dictated.
It is the latter element that was less typical in the South (they say). As a rule, they simply were not as ambitious for gain or advancement. It's all in "Albion's Seed," among other sources.
I think more recent scholarship throws such assumptions into question, and paints a picture of the South as highly energetic, ambitious, risk-taking and most strikingly, prosperous.
Yes, Southerners had fewer miles of railroad and fewer factories than the North, but only the North and parts of England exceeded them in that.
And no nation, including the North exceeded the Deep Cotton South in (white) wealth per-capita, when you consider the accumulated value of slaves.
One thing antebellum Southerners did freely admit was their own unsuitability for field work in their hot summer climate.
That's why, they said, they needed & could not do without African slaves.
Bottom line: from, say 1830 to 1860 Southern growth in population and economics compared to that of the North, when you equate Southern plantations with Northern factories.
And, as many FR posters have noted, by 1860 at least half and maybe 3/4, of US exports (depending on how, exactly, you count it) were products of the South.
So the South was more dynamic that perhaps people like your Thomas Sowell give them credit for.
The less typical industriousness of the south has been attributed to a number of public health issues that were resolved in the first half of the 20th Century. However, the real impediment to industrialization and commercialization of the southern economy was the climate. This was not addressed until the widespread availability of air conditioning in the 1970s. It is no coincidence that the rise of the south took place about that time.
One can generalize that the warmer the climate the less industrious people are. It’s not a hard and fast rule but the idea of a siesta disappears for those who have snow to shovel. This observation isn’t unique to the US. It’s noted worldwide.