Brian grasped the doorknow principle perfectly - he'd get up on his back legs and put one front paw on each side of the knob. If he'd been able to get any traction on it, it would have worked.
If he had an opposed thumb he could turn the knob.Our non-Maine Coon foundling that lived to be 20 years old could open doors. He figured out how to slide the sliding doors; he could hit bi-fold doors just at the right point to make them pop open; and, he would do his best to turn doorknobs.
Cats are remarkably intelligent ... especially when they have intelligent owners who communicate well with them.