I can see the '15s being muddy' apply if they are a high qts speaker being loaded into a too small of a box, which I bet did happen in the past, or high frequency response that drops off quicker off axis due to larger cone diameter.The bass focal length stuff, is just plain wrong.
The 10s being quik, and 15s muddy, is a generalisation, than can apply. The under powering thing, came from back in the day, from PA systems, when amps were best to be over the total wattage of the speakers used. This was to enable to get full SPL from said speaker system, WITHOUT the amp going into clipping, and destroying the speakers.
Interesting backstory in the under powering myth. Clipping amps is definitely a problem, and I can see how it turned into the story we know today.
Maybe it's best to say that some of the audio myths are oversimplifications and partial truths.