How long do they have to keep him there when they agree he did nothing wrong? How come the other guy, a cop, can make a false report?
I'll answer your two questions first, then I have more to say on this later.
1) How long can they keep him there?
As long as it takes for them to complete their investigation.
2) The police appear to understand that he had not done anything wrong in terms of being out early in the morning delivering newspapers. But the radio call was about an alteration, or words, which suggested somebody had been threatened. So, the police are investigating that allegation.
3) How are the officers certain the person who made the call was making a false report,,,, the part about threats? They cops do not know that for certain, and the hostile demeanor of the driver right from the start isn't helping much to assure the police that this man did not make a threat. The man is very angry, almost not quite to the point of boiling over for being contacted by the police, so maybe some anger issues there????? Angry people do sometimes make threats.
Now my opinion.
First off, the police officer doing the investigation was very professional, that is exactly what we want to see. He calmed the driver down with his voice; threated the driver respectfully, go to the bottom of the story, and nobody was hurt.
The driver was angry and excited, and to me seemed to suggest that HE had some kind of anger issues, or hyper sensitivity to anyone making an assumption that he seemed suspicious, and then blamed that only on race. Which I don't see as the case. Of course some unknown car driving slow down a street in the middle of the night looking at houses might seem like reasonable suspicions and cause a resident to have a second look. Or it could have an explanation, like somebody delivering news papers, or an Uber driver, or just somebody lost. But it a reasoable suspicion to maybe want the police to be called to find out, given the number of car thefts, catlytic converter thefts, Amazon thefts..... So, why doesn't the Black guy understand that possibllity too?
It is always important that all of us consider how others may perceive us, especially at night, especially under circumstances like this.
There is no way of knowing for certain if the off duty cop was lying, or the delivery driver was lying. The delivery driver never admitted to making any threats, even as he did admit he had "a conversation" with the other driver, which to me sounded intentionally vague, and I suspect he left out maybe a part where he was angry and came off threatening. But he never admitted that, and the police have nothing left to go on at that point, and a that point their suspicions are satisfied and the driver is free to go.
BTW, what is so wrong with calling the police if you believe something looks suspicious? But the Black driver is convinced that anyone looking his way is only seeing race. Maybe he has had bad experiences, but allowing himself to angry to the boiling point is NOT worth it, and thank God he didn't make any wrong moves, and thank God those officers were professional and intelligent and capable to deescalate quickly.