You're surprised that a middle-aged man from Texas in the early 1960s, infamous for his profane speech would use that term in a private conversation?

Nor does it make sense to single out America for an aberration imposed on it externally, before there was even such a thing as America. IIRC, the article references the slave population of Virgina, but the colonial legislature in Virginia actually outlawed slavery at one point, before being over-ridden by the crown. Britain rightfully deserves a lot of the credit for stamping out the slave trade worldwide, but they were not only the ones who imposed it upon the American colonies, they also profited by it long after it had fallen out of fashion in the British Isles. Thomas Jefferson's original draft of the Declaration of Independence cited that imposition of slavery among the enumerated grievances against the crown, before it was excised for obvious political reasons. The entire plantation system was not particularly profitable for the plantation owners, despite their superficially lavish lifestyle (one detail that is generally overlooked but accurrate in 12 Years A Slave, is the apparent tight finanical constraints under which both Benedict Cumerbatch's benevolent planter, and Michael Fassbender's walking shitstain suffer), as they were in perpetual debt to British merchants & bankers. Though slavery is the major issue behind secession, the actual political crises that raised the idea in the South originated from tariffs. The South was vehemently opposed to tariffs, because their planter class survived on maintaining a trade relationship with England. Absent that, or facing reciprocal tariffs on cotton, they faced financial ruin. Among the issues contributing to George Washington's manumission of his slaves (and dedication to the cause of independence) was his discovery of how he was being victimized when he married into that class, and took over Mount Vernon from his wife's family. He switched to more profitable, less labor intensive food crops and weaned the plantation off the whole cycle, but was left with a large number of excess slaves with insufficient labor to offset their upkeep, whom he lacked the right to dispose of by the unwritten rules of society, or the inhumanity to liquidate as superfluous assets.

The point is that slavery was as much an affliction on the US as a sin committed by it, and was not remotely unique to this country. Nor is the mistreatment of ethnic minorities, or numerous other (legitimate) grievances of black Americans.

And before. There is a reason why they lost.
That's a bullshit, subjective term, not an empirical fact. Black slaves were "prosperous enough" especially compared to contemporary Irish peasants. The typical black slave had better clothing and shelter, and a more substantial and balanced diet. The joke about an Irish seven course meal consisting of a six pack and a boiled potato was not too much of an exaggeration in the early-to-mid 19th century. The free people in the South were the poorest in the country. Their land was rough and badly farmed, and many things that would almost be considered staples in the North, like butter or cheese were the stuff of rumors to most southerners. Education was lower in the South as well, and the infrastructure was greatly inferior to the rest of the country. Bridges, canals and highways were all far more common in the North.

The cotton might have been a significant export, but its importance was in the first place exaggerated to justify slavery, and in the second place, still had nothing to do with the North. The rest of the country profited very little from the cotton exported by a numerically tiny class of planters, and as I referenced above, the revenue from those exports was tied up in a cash-crops-for-manufactured-goods trade cycle with England. The rest of the country kept trying to raise tarrifs on those manufactured goods, both for revenue and to protect American manufacturers. The money from cotton was spent abroad, and did not enrich the country.

Which is why the majority of that economy abandoned it? That role is generally exaggerated for political grievances. While many colonies may have allowed slavery (or simply not felt the need to outlaw something that was not an issue, in addition to not being legally able to outlaw slavery, see above re: Virginia), they were distinct and seperate political entities, whose trade was regulated and curtailed by Britain, for British benefit. New England did not trade much with the South, as England prefered to be the middleman in all colonial transactions. Massachusettes and Virginia traded their respective produce to England, and bought goods from England, almost exclusively in each case. That state of affairs continued after independence and the ratification of the Constitution (a decade separating those two significant milestones on the road to the contemporary unified political entity that is the USA), with, as I referenced above, the Northern states attempting to stress self-sufficiency and internal trade and manufacturing, and the South opposed to all attempts to protect domestic industries or curtail imports.

One more time: The slave-owning portion of the United States attempted to secede from the union, because the rest of the American economy & political was not only independant of their slave industry, but inimical to it. The economic interests of the vast majority of the country were in directions that would have rendered the plantation and slave system obsolete or noncompetitive. It was not because they were prospersous, wealthy and trying to shed the freeloaders who insisted on imposing their inconvenient ideals on everyone else (like, say in Belgium today), but because they so no other recourse to hold onto their lifestyles. Cotton (or rice, tobacco or indigo) was not enriching the country. It was a substantial crop, which brought in a lot of money, but not necessarily much profit, especially with the high overhead. Note that even after the war, when sharecropping became the standard practice, the cotton-growers were still not sufficiently prosperous to spark, much less carry, a southern economic rise, despite their well-known practices which for all intents and purposes did not represent a significant practical improvement over slavery.

Which were these? Where, in Europe or North America, was mining or manufacturing done in any significant quantity by actual slaves (as opposed to low-paid white people) in the last 400 years?