> Large grocery chains don't have anomalously high profits

Define "anomalously" What do you mean by this? You'd admit that large grocery chains do have high profits right? Even with "razor thin" margins.

Large grocery outlets do love to claim that their margins are razor thin yet somehow each of them are some of the wealthiest in the nation. How can that be if they are so good for the customers?

> its not clear what anti trust even means here.

Price fixing [1]

> There's such a strong incentive to defect and secretly sell more under the table. Even OPEC could not maintain one.

OPEC is the exception, not the rule. And enforcement is a lot easier when there are fewer players. The reason OPEC has a hard time enforcing it's prices is because there are are too many players. Were they standard oil, they'd almost certainly be able to maintain their prices and discipline.

Cartels very rarely dissolve on their own. The free market naturally tends towards consolidation and cartels forming.

[1] https://en.wikipedia.org/wiki/DRAM_industry_price_fixing