Iceland's power is hydro. (It's only ~26% geothermal)

The Aluminum smelting is done on hydro not geothermal.

Iceland is small compared to the USA or Indonesia for geothermal power. Large per capita, but Mexico has more geothermal power.

https://www.thinkgeoenergy.com/the-top-10-geothermal-countri...

(Iceland also has geothermal heating for housing which works well)

Usually its done with standard hydropower not geothermal, lots of aluminum smelting in Quebec. China produces like 10x more than the next highest producing country.

Not all thermal energy is created equal. The temperature level plays a huge role, and I imagine geothermal is on the low end. Suitable for household heating and the like, but not for smelting metal. From hydro, you generate electricity, and that can be converted to thermal energy on very high temperature levels.