Read the article more carefully. The police got a report that a white man and a black woman were having sex in a car. There was no evidence that it was this white man and black woman, and they weren't in a car when they were stopped. So no probable cause to arrest them.
Maybe they got out of the car when they were approached by the police. 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.
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. The caller may have merely said, “a man and a woman”, especiallly since this is a gay-marriage state where any combination of sexes can make out, again, just a description. Then the 911 operator may have asked about their race(s) or even their clothing just to get a description.