Nope, it’s not a pledge; their policy states:
Costco policy prohibits firearms to be brought into the warehouse, except in the case of authorized law enforcement officers.
Since this does not conform to the requirements outlined by the State of Texas to meet effective notice it means absolutely nothing to those with an LTC. Might as well say all patrons must wear pink undies, unless they’re going to stop everyone at the door to check (either for pink undies or a gun) it’s just word with no teeth.