I though the theme was that Pietro was a self-deluded drug addict who couldn't handle being ordinary to the point that he endangered his own daughter (and killed one of the inhumans). the repowering mutants was his way of covering for what he had done in the name of regaining his powers. the story works because it's Pietro. he's impulsive, selfish, and arrogant. the source of his powers was irrelevant. he could be some test tube mutate like the genoshans or wundagore experiments. there are no established rules for what terrigen exposure does to someone with artificial augmentation. the rules were already altered to allow Luna a power upgrade.