The primary cause of hypertophy is mechanical tension, which is achieved by creating resistance against a muscle's force. Flexing is an isometric exercise, but doesn't provide much tension and therefore doesn't stimulate a lot of muscle growth. Climbing offers a ton of tension, both isometrically by holding up your body weight and isotonically by pulling up your body weight. Climbing is usually significantly anaerobic and will damage muscle fibers, which are two other stimulants of muscle growth.

Cardiovascular exercise is good for strengthening heart and skeletal muscles, but not for getting swole. If combined with good diet it can help reduce body fat percentage to achieve a more cut appearance.