The point of flushing as you said is to remove the nutritional buildup if you will, it removes the excess salts to avoid an over abundance. Overwatering is just that. Imagine basically flushing ur plants EVERY time you watered. This is essentially overwatering. Its watering enough or frequent enough to cause an exclusion of oxygen to the rootzone. A flush is a good and healthy monthly practice. Overwatering should be avoided at all cost