It makes so much sense. I have never been near a gym in my life so this is an uneducated observation, but in a gym you lift weights in increments and through a certain range of motions to gain muscle mass. The motions and increments make a difference to where the muscle is built on the body and also train muscle memory.
Where are these motions ever likely to be closely repeated on a rugby field? Surely you would be so much better gaining muscle in random motions and situations to more closely represent the unknown efforts that are going to be required on a rugby field.
This is also in reference to the AB training. these guys would have a periodised program year round. The major part of the Hypertrophy and Strength phases would be carried out in Off season and early pre season. These guys would have big core lifts being Squats, deadlifts, bench press and military presses.
Also all of these lifts plus other olympic style lifts like power cleans and snatches have direct carry over to rugby. All require very strong core and stabilser muscles and depending on your focus will build strength and power in different cycles. Isolation excercises I imagine would be a very small part of their time in the gym.