Instead of thinking of this as a "rule", it is more helpful to think of it as a "guideline". In order to complete long sections of the AT, it may be necessary to break this "rule", simply because what you will need in the way of gear, fuel, food and water will exceed 20% of your body weight.
If it makes you feel any better, just realize that bodies vary widely in height, weight, metabolism, lung capacity, fitness, and so on. Hikes vary widely in length, elevation gain, weather, and trail conditions. Positing a strict rule based only on a single variable, a hiker's weight, cannot possibly account for all the important factors that go into a successful backpack.
So, consider it a suggestion and a starting place. Actual hiking will teach you just how far you can bend that "rule" and still meet your goals, whether those goals are making big miles, or just having a great time. Once you've begun your test hikes, things will come into much sharper focus.