No, it's not really due to the lowering, although that may contribute a little, it would be true even without that. It's due to the angle of the leg remaining mostly vertical - the heel doesn't become the contact point unless/until the foot gets ahead of the body, which in a well executed heel lead step it does by only a small amount at the very end of the stride. This also implies that the ball of foot will lower fairly quickly, becuase the body is already nearly over it and so will arrive quickly once the back leg is finally departed. Really dancing well is far more like walking normally than most of us realize - we have to learn how to get back to an ordinary action despite the presence of a partner and pressure of competition - adjustments that ultimately be made consist of doing more of what is an ordinary walk, not doing something different as so many spend their first years attempting.