While no one can dispute the YF12A's speed, information from the link you provided doesn't mention it ever being used as an actual fighter plane. And except for the few mentions of missle tests against drones, there's no mention of any armament or weaponry.
Because of its speed and altitude capabilities, it's primary function has been to do radar and photo recon missions over the Soviet Union, China and SE Asia, where it has been a total success.