There are several factors speaking against the casting theory.
1. Molten metal is what is known as a contaminant, and impossible to work with as it does not have an appreciable volume. It is not physically possible to pour molten metal on molten metal to make the layer of molten metal thicker. This seems like a very formidable obstacle to creating useful objects out of it.
2. Likewise, it is impossible to dig smaller holes than the size of a dwarf, so this would yield an anvil approximately the size of a dwarf.
3. Anvils must be forged, not cast, as any dwarven smith will tell you, to be made. If metal items could be made by melting metal, why would the anvil exist in the first place?
I think this allows us to safely determine it was not "cast", unless the universe was somehow different at the time. Now, if the first anvil turned out to be made of wood during a strange mood, it would very likely be an artifact. This would of course make it physically immune to heat, but not to catching on fire. Possibly it is still burning somewhere after being used in a magma forge, and could be found following an unexplained trail of smoke?
Actually this could yield some information on the prehistoric dwarven condition. If it were made of dragon bone, that is one thing. If dwarf bone, we would take that as evidence of societal problems and unrest at the time, possibly even deaths (of beloved prehistoric lye maker?) causing unhappiness... in a time supposedly before time.