I heard this thing on the internet at one time that racism wasn't a thing until the African slave trade.

Sure, before then people did get discriminated against but not in the same way they are now. People weren't discriminated against for being black for example. In Rome black people could do all the things that white people could do and interracial marriages weren't seen as taboo. That doesn't mean the Romans didn't discriminate, they might have but they probably discriminated based off of something else.

it seems like they modern idea of race is a modern invention invented by people who partook in the African slave trade to justify slavery. after all, how can you partake in something so cruel as slavery, especially chattel slavery on another human being? if you invent race, and then say that the races are just different, and that black people are inferior to white people then you have a story.

This also means that white people before the slave trade just didn't exist because before then Germans would not have even considered French people to be their allies. They would have been at each other's throats.