It's confusing to me too. I understand why King of the Swamp doesn't work, it says it only substitutes for a specific fusion material whose name is on the card. But usually a fusion monster card will specify "must be fusion summoned with the above fusion material monsters" if it is to restrict what you can use.
you cant use it because its say hero monster fusion sub is not a hero monster if the fusion card say something like "elemental hero neos + hero monster" than you can use fusion sub to replace neos.
Confused guy
Ojama King