I think that refers to cheese, not to powdered milk (although I am hearing it is no longer true about cheese, either). Why should there be any difficulty in making unkosher powdered milk from ANY source? The water in the milk is completely evaporated through a process, and what is left is the powdered residue. It is reconstituted with water when needed (i.e. to feed a baby horse or csmel).
I don’t know about making cheese from unkosher milk nowadays, but you’re right about powdered milk. The Chazon Ish says that, and even the Har Tzvi who is lenient has other reasons, as I mentioned earlier.