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.
https://en.wikipedia.org/wiki/Fugitive_Slave_Act_of_1793
https://en.wikipedia.org/wiki/Fugitive_Slave_Act_of_1850
"It required that all escaped slaves, upon capture, be returned to the slave-owner and that officials and citizens of free states had to cooperate."
Boston's police department was founded in 1854.