It's definitely part of the infrastructure of nuclear weapons. That's why it was hacked in the first place. They were making weapons grade nuclear material.
So is a steel plant though. Keeping a missile from being hacked is much different than something in the supply chain.
Considering the number of steel plants and the relative cost of the material, that’s not a very good comparison. The whole point of Stuxnet was that the facility was a critical single failure point in the supply chain. When it went down, it disrupted the project for years. I don’t think the same dynamic holds for steel manufacturing. If you think they’re comparable, try getting into Oak Ridge or similar; I suspect you’ll find it very well guarded compared to a steel plant.