Fusing into iron is different from fusing with iron. Binding energy is a negative energy value; iron having a high binding energy means it is a low energy stat

Fusing into iron is different from fusing with iron. Binding energy is a negative energy value; iron having a high binding energy means it is a low energy state, so adding zero-binding-energy hydrogen to that system and decreasing the binding energy increases the potential energy of the system from the low value of iron.