But neither of them actually looked like they would score, that was the issue
The problem is that a normal side would see Stones charging up the pitch or taking shots or running into the box and they'd go and defend him. It's the whole reason Pep likes CBs who play out from the back - you come forwards and someone has to deal with you, and that leaves somoene else free up the pitch.
But because Bielsa is such a nut, and has such an obsession with man to man marking, no one is allowed to leave their man so no one ever left someone to confront Stones.
Unfortunately I'm not sure Stones quite came to terms with this. He should have been taking a lot more shots, standing in the penalty area as a striker, because then he'd have forced someone to deal with him and leave another, more dangerous player, unmarked.
That's why Pep subbed Gundo for Ake, because in that man to man system, a CB being the free player is not ideal.