I find it worrying when I read things like this. Perhaps the student mis-read the quote..or didn't read on for the explanation, which can be the only excuse for such a sweeping generalisation. We all like to have nice, simple "rules" like this to tuck away in our memory banks. BUT they can let us down if we do not fully understand them and use them willy-nilly.
When you begin to include people in your pictures, there are lots of things to bear in mind. The most important one is YOUR EYE LEVEL. This will vary, depending on whether you are standing, or sitting, to paint/sketch. Then, after that,the relative heights of the individuals in the scene can be taken into account - children, for instance, will be shorter than adults obviously......but relative heights MUST be a secondary consideration. Understanding your eye level fully is essential. To find it, put a small sketchbook horizontally onto the bridge of your nose, make sure it is level, not tipped up or down, and look out across it. Where it visually "touches", that is your eye level.
Here is a sketch I found in an old book - look at the clothes, you will get a sense of how old the book is!!! I was most amused. However, It really explains, visually, why the eye level is so important:
Keep the eye level firmly fixed in your mind - and perhaps mark it on your canvas too - you should find it helpful.