"Maybe" isn't probable cause.
Or maybe the caller gave the license number and description of the car, and it matched when the guy gave his ID. So your “no probable cause” theory isn’t holding up.
According to the article, all the police had was a report that a white man and a black woman had sex in a car. So, without a further description, that is not probable cause to stop any black man and white woman they happen to see in the vicinity. In fact, according to the article, the police eventually concluded that no crime had been committed by anyone.
Also, if the caller described them by race, it may have been just that — a description to identify the individuals, not an intentional act of racial intimidation.
No one is accusing the caller of racial discrimination, and I, for one, don't think the cop is guilty of racial discrimination either. But California does not have a "show your papers" law (some states do), so the cop had no right to handcuff her for refusing to show ID.
People reported sex in the car in public. Sgt. Parker responded. She refused to give ID and left the scene. Another car arrived and a female officer cuffed her.
The police did not determine a crime had not been committed. This ended with no citizen who witnessed it wishing to make a citizen’s arrest.
I remain unconvinced at the conclusions to which you are leaping.