I know a little bit about mic'ing pianos, and I disagree strongly with that statement, unless I am misunderstanding what you mean.
If you are looking for a natural piano sound, the only time hard panning will give you that result is if the mics are in some sort of close stereo configuration, like a coincident pair, for instance. Then hard panning should sound right.
However, most of the time, if you mic the top and bottom of a piano and pan hard, you will end up with a piano that sounds 20 feet wide.