The pattern was exploitable only on the specific days that Jane Street was allegedly manipulating. How would you have figured out, without counterparty information and before noisy sales start dragging down the index, that day X is a manipulation day?

How would you have identified that there's even such a thing as a manipulation day? Do you have a model that tells you the objectively correct number of days a non-manipulated index should be lower at close?

Simply look at the market patterns and make a profit, duh! Why doesn't everyone do that?!

Usually, everyone does do that, which is why only hard-to-detect patterns remain profitable. Not something obvious like "buy options in the morning and sell in the evening" as in this example.

But maybe Jane Street only traded like this on some days, so you would need to know whether they had done so before you could hope to exploit them.

> Simply look at the market patterns and make a profit, duh! Why doesn't everyone do that?!

I know your question was sarcastic, but not everyone is able to see patterns. And not every trader is as good as the other. Hence winners and losers.

Yes claim is price is high at open low at close. Seems pretty straightforward.