Ma‘siyah is committing a prohibited act while still acknowledging its prohibition. Istihlal is treating an unequivocally forbidden act as religiously lawful. These are not the same.
Someone who sins through weakness yet knows the act is wrong and needs repentance differs from someone who denies an established prohibition.
The distinction does not make sin harmless; it demands both condemnation of wrongdoing and care in judgments about faith and disbelief.