The video shows his heel with studs up making contact near the outside ball of his ankle near enough to the achilles but is subjective from the angle they give us. Ref was on on the side and had a better view .
Red card was strange but I think it has to do with targeting with intent the unprotected achilles area similar to intentionally kicking someone in the crotch or head,etc...
The foul stopped play technically and he did look over at the player rubbing his face-probably thought he was playacting and playe d on! Good ref and he applied the rule