It comes from partisans and us history.
While the police in the us have been infiltrated by the far right, there are other people willing to get in there and observe.
Secondly, look at uprisings in the past and present the cops are the first line of offense against the people. That's kinda what the post is about, cops being used as a tool of the state to oppress workers. Look at some of the union wars from back in the day, it was cops that were the first line used to try and beak protests and disrupt unions.
If shit ever pops off, it takes time to mobilize national guard, and even longer for other forces. But cops are in every town, every city, every county, and they've been militarized heavily. They've been trained to think of themselves as warriors against the populace, wherein the populace are the bad guys, and they're the thin blue line.
This is backed up by people that are cops, and know damn good and well that the infiltration by the far right is in there, waiting for when it's time to stop pretending. There are way less people in there on behalf of left wing groups, but they exist.
But there is a limit to it. Once there's enough power secured, the pretense goes away, and the police become an army in every way that matters, an ss, a gestapo. At that point, we're deep into dystopia and revolution is no longer a realistic option, it goes to resistance and a gradual push, asymmetric activity.