Yeah, you're right. I'm just talking about spherical chickens here. In real life, if your bank quotes you an APY they've already annualized it. I just think it's interesting to think about how in a perfect world, the rule is actually really good. Reality is just way less elegant than a classroom, unfortunately.

So I should have been clearer that I'm not criticizing the article, just offering another perspective on the how and why the rule is imperfect, which the article acknowledges as well.