In all the newbie lifting programs (Starting Strength, StrongLifts etc) you'll see that untrained individuals can achieve almost linear growth for the first couple of months. Their size are definitely not growing linearly though. Most of the gains is from what they call "Motor Unit Recruitment" where your brain learns how to activate muscles you already have to complete increasingly heavier lifts.

In that sense the experiment here makes sense. You're just making sure you still remember how to fully utilize the muscles even if they are in cast. I doubt there's any difference in terms of muscle atrophy (and of course don't believe just thinking about it can lead to hypertrophy).