The entity now called Costco was called PriceCostco after the 1993 merger. That they ultimately chose the Costco name doesn't change the dual heritage; founders of either have a fair claim to being founders of the combined entity.
But even further: key founder of pre-merger Costco James Sinegal got started in retail working at Sol Price's San Diego-based FedMart and Price Club. He then copied the PriceClub membership/bulk/employee-development model starting from Seattle. (A friend working for pre-merger Costco at the time described it as an almost-friendly rivalry, with each racing to expand the shared model from their home cities into the rest of the US.)
So the PriceCostco merger "got the band back together", Price Club was the older and model-foundation entity, but clearly both teams contributed enough to be credited as "founders" of the current behemoth.