One, even if all police in the U.S. did start as slave patrols it is a textbook case of a genetic fallacy.
Two, your article discusses several origins of police forces in the US. In Boston it had nothing to do with slaves because Massachusetts was not a slave state when they created a police system in the 1830s. And since Afroman was raided in Ohio, also never a slave state, it does not make sense to carry over southern slave-catching history into modern police culture.