If by “small part” he means “primary cause of” then he’s correct. ;d
The war was about the EXPANSION of slavery into new states and territories.
The South took the position that its recognition in the Constitution meant slavery was applicable in all states and territories, whereas the North took the position that the Constitution recognized the ‘peculiar institution’ only in states where it was already a defining feature.
It always puzzles me why it is ignored that the Abolitionist Movement was a novel interpretation of Christianity by the Second Great Awakening.