Elaborate? I point out the fact that:

* The principles expressed in founding documents contradict slavery.

* Some States banned slavery less than a decade after the country's founding, and within forty years the majority of the country by population had banned slavery.

* The contradiction between the founding principles and slavery ultimately led to a Civil War where the former emerged victorious.

* The United States' did not take an abnormal amount of time to ban slavery at a national level. And in fact, some States in the US were the first governments to ban slavery in the entire New World.

I'm pretty much at a loss as to how someone can come away with this comment thinking that nothing in my comment is refuting the claim that the US was founded on slavery.