Yes, sometime within the last 5 years they broke their long partnership with AMEX. It seems to be Citibank now.
Making 4% back on gas is mega killer though.
That said, I do kinda miss the Amex and its automatic extended warranty.
[0]https://money.cnn.com/2016/03/31/pf/costco-visa-american-exp...
The Costco Visa also include an automatic warranty extension of 2 years beyond manufacturer.