while hashem himself and his essence is not moser nefesh, the way that he acts towards us is such that if the action were to be done by a human, it would be called mesiras nefesh. it’s the same deal with all middos of hashem. after all, hashem is not actually merciful, slow to anger, loving, kind, etc (see beginning of moreh nevuchim). he has no emotions, and therefore can’t really be moser nefesh or possess love/mercy/kindness the way we do. rather these middos are descriptors of hashem’s behavior and actions, that if they were carried out by humans, would be called as such.