I disagree with the conclusion. It is wrong because the ones who enslaved the black men were black men and black men owned slaves as well. Racism was, obviously, essential (for slavery to exist the slave owner had to view the slave as inferior). But racism was not the cause of slavery (people did not stumble upon a black guy and say "hey, he's another race.... Let's make him a slave and see if that will help our personal economies".
The 3/5 law was the same. It was the racism of the North (it was the North taking advantage of a race for their benefit). But race, again, was not the cause.
The point at this stage is not which caused which; slavery or racism, who sold who into slavery, or the 3/5th clause, etc. This is about the
intentions of the Confederacy, and why that former regime and its emblems are offensive to people today.
In other words, it's racism (
together with the slavery it intended to justify) as the basis of
the Confederacy's desire to secede,
not "racism as the basis [meaning "
cause", as you seem to be taking it] of slavery" [see below]. They didn't care who started slavery; they just didn't want it to end, and had built a whole belief system to
justify it (which would actually suggest your point "racism was not the cause"; it came afterward, as the
effect). So that's not what's being argued against.
When the African merchants sold slaves, most captured in war, "At that time, there was no concept of being African – identity and loyalty were based on kinship or membership of a specific kingdom or society, rather than to the African continent." (
The slave trade's effect on African societies - Implications of the slave trade for African societies - Higher History Revision - BBC Bitesize) It was once in Western captivity, that this thing called "race" became the deciding factor, and people discriminated against for it (whether by slavery, its replacement Jim Crow or anything else).
That's what I meant by "racism as the basis of slavery"; "basis" being not a timelike meaning of causation, but as the
already established [by
then]
rationale (justification), that the people were inferior. The Confederates weren't protesting "We bought those slaves from the African merchants, and [no matter who they are], taking this property from us is just one of other more important things we are seceding over". What they said was was that "we
are fighting to maintain the heaven-ordained supremacy of the white man over the inferior or colored race", and "the negro is not equal to the white man; that slavery, subordination to the superior race, is his natural and normal condition".
So no; no one here is saying "racism" came first; nor does it make a difference at this point.