This is only true if your feet naturally point the same direction as your knees. Most people's do. Mine do
not. And while my particular condition is uncommon, being "duck footed" or "splay-footed" is not uncommon. For people with this structure, your "rule" does not apply, and in fact can damage the knees!
Yet I had one instructor who stubbornly insisted that the the human foot
always aligns with the direction of the knee and tried to force me into the traditional stance, nearly wrecking my knee joint. When I showed him my problem and explained my medical history, he insisted that I was the one being stubborn.
I came to the conclusion that while he was a great practitioner, he was a crappy teacher or coach
for me. I respectfully stopped training with him. A shame really. But your health must come first.