Given professional policing was invented in London and copied by big American cities...really really hard to square that with that line.
...I was gonna say.
IIRC, My criminal justice textbooks enjoyed referring to the NYPD as the first professional police department in the US. The New York City Police Department was founded in 1845 (and was preceded by other organizations), a mere decade or so after the UK established their system. I highly doubt that the first priority of an 1800s NYC police force was capturing runaway slaves from several states away.
Also, per Wikipedia, Peter Cooper, noted as
an abolitionist in the years prior to the Civil War,
was tasked with drawing up plans for the department. I assume he would not have done this if it was with the purpose of slave catching.
