Its never been said that Death's Scythe could kill anything. It was only implied that it may even kill Death himself and it did. Death also did not say he'd kill God, he said "I'll reap him too" which does not exactly mean the same. Billie said the same thing in S11, that he was moments away from reaping him. So far, the only confirmed thing that can kill or hurt God is Amara. Rest is just assumptions.