Can't read the documents you posted.
They can tell you no guns because it is private property. This is why I let my membership expire. I didn't know this at the time but I have heard of people asking for a refund of thier membership fees because it is not (or wasn't when I signed mine) stated in the agreement that firearms are not allowed on COSTCO property.
If I were you, I would get my money back and tell COSTCO you will be taking your bussiness to a company that doens't take away your constititional rights.