Summarizing: Costco paid tariffs on things, and so they raised prices accordingly, passing those increases on to their customers. The tariffs were recently ruled illegal, so Costco is suing the government for reimbursement of the illegal tariffs, and have said they'll pass the repayment on to customers by lowering future prices. A group is unhappy with that approach and is suing to force Costco to reimburse customers who paid the higher prices in the past, not pass the money through future sales.
I'm guessing it's much, much harder to figure out who paid the higher prices in the past and how to get the money to them. Personally, I'm just happy Costco is trying to force the government to reimburse the tariff money. I could see them using the refund to lower or eliminate membership prices for anyone who was a member during the time the tariffs were in effect.
Yeah, I'm sure they have data on sales by membership. But it would be complicated by the number of people whose contact info is out of date, the ones who are no longer members, mailing checks that don't get cashed, etc. For sure not impossible, just a lot harder and more expensive.