Technically, he is right in “The Civil War” being avoided BUT it would have just deferred conflict in the long run. There was no way the US would have continued with slavery. Absolutely zero chance due to an economic perspective.
The federal government would have 100% stroked a deal where select southern states could have kept slavery but the other states and especially the newest states would not be able have slavery. Not being able to negotiate this is what actually led to civil war. The confederates did not want to negotiate.
The North is industrial and made products to sell and export. The south used slavery for agriculture. If the slavery states would use that free labor to go industrial it would be a DISASTER for the slave free sates. Conflict would then arise to make sure the south no longer has that “economic advantage” over the north.
tldr: Any market with legal slavery will go “capitalism full bloom” and create a market where free Americans have no chance to compete.