It's use in metallurgy isn't heat, but chemistry, removing excess oxygen atoms from iron ore, effectively reversing rusting.

https://cleantechnica.com/2023/03/21/steel-is-a-major-climate-problem-but-we-have-proven-tools/

For the why on hydrogen.

https://cleantechnica.com/2023/05/16/what-drives-this-madness-on-hydrogen/