Good article. Maybe it’s the “all” and “only” that sets some people off. The problem is that some states didn’t secede when it was all about slavery, but waited until war had already begun. I’d just say secession was about slavery and leave it at that.
Sorry, I'm usually so careful about saying "Deep South secession was all about slavery" to prevent any confusion over Upper South states: Virginia, North Carolina, Tennessee & Arkansas.
Those states all voted against secession, and only switched after war had already begun -- when they felt forced to chose sides.
Virginians, for example, can and do claim they seceded due to "oppression" as specified in their Constitution ratification statement.
But they also chose secession after the Confederacy had formally declared war on the United States, so they well knew from the beginning what they would be getting.