"9. Fingers outstretched toward an opponent’s face/eyes; a. In the standing position, a fighter that moves their arm(s) toward their opponent with an open hand, fingers pointing at the opponent’s face/eyes, will be a foul. Referees are to prevent this dangerous behavior by communicating clearly to fighters. Fighters are directed to close their fists or point their fingers straight up in the air when reaching toward their opponent."
He wasn't reaching forward. There was no forward motion of the arm from Song.
Hand was stationary ready to parry, Cejudo drove his face into it as he punched.
Not a foul, read the rules again.