Costco's contract with Interstate Battery System could specify that they get Clarios batteries. I have never seen anything else at Costco.
Costco had an exclusive supply contact with JCI before, for its house-brand Kirkland batteries. It was even noted on the promo materials.
I suspect they're not doing anything more than licensing the current brand name from IBS.
Costco Interstate batteries can only be handled and warrantied through Costco, not IBS or its dealers, and with its own considerable buying power, has no reason to employ another middleman/reseller to procure its supply of product.
And to me, it doesn't make a lot of sense to even do that type of branding.
Sure, the Interstate brand has some equity, but I'd argue it's no stronger than DieHard, or what the others possess, to warrant such a deal, not to mention Costco's own strong brand equity.
Or that brand supercedes things like price, warranty, and convenience, for most shoppers, for what are commodities produced only by a small handful of companies. You see it here, even among those who have more than a superficial knowledge about the product and market. Price, fit, and need, at the top of the priority list, not brand.