My late cat Raedthinn also knew how to open doors. He figured it out when I briefly lived in an apartment with door *handles*; he watched me use them, figured out what was going on and would reach up and pull the handle down to open the door. When we got back to door *knobs* he knew how they worked, but he could rarely get enough leverage on them to open doors. But “rarely” isn’t the same as “never.”
If your cat is opening doors when you don’t want him or her to do so, don’t get mad at the cat. Cat’s doing what a cat’s *supposed* to do. Be mad at yourself for being outsmarted by a critter with a brain the size of a walnut.