You are correct in a true PI situation that who initiates contact first should get flagged. No arguments from me.
HOWEVER, the guy who made the statement that "the defender made contact first" on a receiver the ball WAS NOT GOING TO is not valid in this situation. The play was clearly a pick play. it was a trips right (bunch formation) and the two flankers' job was to clear out defenders and create a 1 on 1 to the slot (the guy closest to the QB in the bunch formation to the right) who was running an out route. See below:
You can see in the second picture below that the two defenders who's job was simply to clear out their defenders from the play, literally run into their defenders to pick off all 3 of them (the guy closest to the QB got flagged... it really should have been the far guy...). Either way, they are ENGAGED in a block with the defender before the ball is ever thrown. Now, if the receiver stayed in the back field (i.e., not move beyond the LOS), this is an entirely legal play. You would know this as a screen. HOWEVER, the receiver went beyond, and subsequently caught the ball beyond the LOS, meaning the receivers are illegally picking/blocking defenders prior to the pass being thrown. This allowed the slot to come WIDE *** OPEN in the endzone. Receivers don't just get open like that... not at this level.
That was my point... that it doesn't matter who "initiated contact" here, the receivers cannot intentionally impede the defender's progress toward the ball on a pass play beyond the LOS, which is what happened.
Pick plays are legal so long as they don't look like "pick plays..." the receivers must be somewhat convincing in the illusion that they are simply running their routes. That did not happen here.
ETA: Found a GIF of the play.