Repeated PD is very different. The whole point of a cartel is that everyone inside is incentivized not to cheat, and it works as long as they agree on what's fair. The problem for cartels is people not in the cartel.
It’s well known that some OPEC members cheat nonetheless.
I'm not fully informed about the secret meetings of OPEC, but I would read things not so much as cheating as that everyone agrees that Saudi Arabia will reduce output - anyone else reducing output is reducing by the amount that will happen anyway because of wells drying up.