any art when practiced correctly is "internal," by definition; relaxed power, focused breath, intention, correct structure and alignment working as a unit, creating power and rooting.
If it is forced, relying on brute strength improperly applied, it is not "external," it is simply wrong.
Usually it is a result of improper or incomplete teaching, spread by guys who "earned" their Black Belts (a beginner's rank) and proclaimed themselves Masters, thus continuing the myth.
The terms, "internal and external," are simply thrown around by modern day "internal stylists," and is nothing more than snobbery.
Good Shotokan is as internal as Tai Gihk Kuen.
"My Gung-Fu may not be Your Gung-Fu.
Gwok-Si, Gwok-Faht"
"I will not be part of the generation
that killed Kung-Fu."
....step.