https://www.ajaonline.org/article/3863
> In July 2014, we conducted a survey of Pompeii’s street network to document traces of iron that were observed on the stone-paved streets, which resulted in the identification of 434 instances of solid iron and iron staining among the paving stones. ... Pompeians were—in addition to using solid iron wedges—pouring molten iron and iron slag onto their streets as a method of emergency repair.
There's a huge gulf of difference between "a civilization is capable of melting iron" and "molten iron is so cheap and easy they used it to fill potholes".
Castable iron: Valuable, scarce, energy intensive, very hard to melt (comparatively), corrodes readily (especially on the coast)
Iron and other metal slag: Byproduct of iron smelting, basically waste, melts fairly easily, was used as ballast in ships, comparatively cheap, rusts superficially but is mostly a matrix of oxides which don't meaningfully react
I'm beyond skeptical that perfectly good cast iron was used to fill potholes. Using slag/dross/failed smelts/other byproducts of metallurgy makes far better sense.
It would be like far future archeologists unearthing rammed earth tire walls and concluding "the 20th century was so efficient at producing rubber they used excess tires as a building material" vs "they used a waste product which would have been landfill anyways".