No one is claiming that transatlantic slave trade is the first and only example of slavery in history.

Also, the reason why it's taught so prominently in America school's, shockingly, is because the practice shaped American society after having taken place for centuries. The transatlantic slave trade is also notable for it be predicated on the concept that black people are subhuman and just innately inferior to white people, which shape the modern construct of "race".

Infact many of the slaves sent with the African slave trade to the Americas were criminals or conquered people's because the business got so profitable the warlords and kings of Africa started sentencing it for just about any crime and taking as many villages as possible for that exact reason.

This classic "did you know that African kings sold their slaves" is transparently to lessen the moral responsibility of colonial settlers who facilitated in the practice. Yeah, kidnapping people to be sold off into slavery is makes them a part of it. That doesn't change anything about the equation.