The perfective/imperfective (the one here) difference. In the original sentence, either they were helping the police regularly, or they were helping - and then we have no idea whether they managed to help or failed, maybe they weren't useful to the police in the end.
If they actually helped, succeeded in it, this would be "pomogli". And would denote a one-time help.
they helped should still work tho as you can always translate PL imperfect by EN simple past
I disagree. This acronym (and possibly even more so your reaction) gave me an insight in contemporary Polish culture and street language, something I would not have gotten from the official Duo course.