FAQ is wrong about Supergirl, the Flash, and Arrow
Its clearly stated during the Flash's appearance in Supergirl (episode 18) that he is from an alternate universe. That he entered Supergirl's by accident. Talking to Kara and friends he learns The Flash, Green Arrow and the other characters from Arrow and The Flash series do not exist on Supergirl's earth. And given his surprise at learning Supergirl is an alien, Supergirl and Superman do not exist in his universe. At the end of the episode Supergirl helps The Flash to generate enough speed to return to his universe.
The episode doesn't call either Earth, Prime or give either a number.