Exactly its an 'ability' to cut steel only
I was already expecting that you could not comprehend what I even meant.
You are correct. Zoro only knows that this technique can be used to cut steel. He has no way of knowing that the same ability is also able to cut something intangible like lightning. There is no implication that it could. Its even counter intuitive since steel is very solid while lightning is intangible.
If you had a scissor that was designed to cut paper, but you end up needing to cut cloth you could deduce that the same scissor may also be used to cut the cloth. In the situation with Enel Zoro has no reason to deduce that Shi shi Son son would be able to cut Enel too.
Zoro in Alabasta is effortlessly cutting through stone. No bladed weapon can do that so easily. He could also turn the sharpness of his blade off completely when he brushes that palmtree leaves. No bladed weapong can do that.
Haki = Will.
Zoros swords following his Will = Zoros swords following his haki.