The "Shovel Principle" isn't a rule of kata interpretation. Instead it is an observation about karate practitioners.
Some of us dig for the truth in spite of everything our Japanese leaders do to obscure it. If you are a digger, then at first you have hope. If you can shovel aside enough of the muck, you think, you will find the truths that you always knew were there.
After you have worn out a few shovels in this pursuit, your attitude changes. Maybe it is just disappointment, or sour grapes, or perhaps just the delayed arrival of good judgement, but you finally realize that there is nothing under the manure pile but more manure. This give us the Shovel Principle:
Shovel principle: Same Shotokan, Different Day.
The Shovel Principle is really just a restatement of Climbing the Other Mountains. If you want to understand Shotokan, you need to study jujutsu (aikido, aikijutsu, etc.,) and combative arts.
In the House of Shotokan, the stairways do not go all the way to the top. To master Shotokan, you must leave it.