Honestly, i don't think this theory is true. As you said above, you used a shiny parents and got a baby shiny at 640 eggs, this rate is actually worse than avarage (1/512) and if you looking into the game machenic,
http://bulbapedia.bulbagarden.net/wiki/Masuda_method. When breeding,a shiny pokemon is treat the same as normal one and if they are different origin then the game will try to generate ten times the personality value. If one of them could result in a shiny then it will be chosen, not based on the parent shiny or not.
Actualy, i already tried breeding shiny using a shiny parents before (a starly) but don't get any shiny over thousand eggs at all