I was taught that wai gong was a very physical/external method of chi development. As in, a lot of physical movement (arm flapping, waist rotation/bending, etc) that forced the blood and lymph into specific parts of the body…hence also forcing chi into those areas as well.
Nei gong, in contrast, was a softer method w/ very little movement if any where the body is put into a specific posture and then the mind is doing the work.
(Nei Gong is training which is designed specifically for the development of muscle groups, ligaments, and tendons not usually under conscious control. This training involves refinement of the basic wai gong skills and development of the connection between mind and body. In the beginning levels of nei gong training, repetitive physical movements are combined with: e development of muscle groups, ligaments, and tendons )
I would say that his understanding is incorrect based on my own experiences and understanding. Specifically nei gong training trains or develops ones shen, yi and qi or sprit, intent and inner energy or air.
Wai gong or outer training develops the bone, tendons and skin. the practices are not interchangeable and are dramatically different in approach.
Confusion arises because most people really do not have a firm understanding or bases of experience in which to judge it. This is amplified by some Chinese masters who are used as spokes persons for a given point of view.