iow, hello99 is saying that the kula for people who only eat CY (and do not eat CS) nevertheless eating non-CY powdered milk, was that the poskim said it is physically impossible to make powdered milk from non-kosher sources.
now, that metzius changed, in that it has become physically possible in more recent years to make powdered milk from non-kosher sources. so the original basis for the heter is no longer applicable, considering the new (and changed) metzius. (and to come up with a new reason for a heter for powdered milk in that they are usda labeled, you could use that same logic to be matir all CS — which some people do, but if you are machmir on CY only there is no more reason to be meikel on powdered milk.
i am just explaining hello99’s point as i understand it. if i understood anything incorrectly, please correct.