Every individual action "contributes practically nothing to global CO2", so with this argument, nothing should ever be done.
But this is a strawman argument regardless: globally, steelmaking contributes ~11% of direct CO2 emissions[1]. Other countries[2] are progressing towards decarbonization. Per this very article, Europe will subsidize initially (progressively replacing subsidization with import tariffs over the next decade). And the whole point of all of this is to "internalize the externalities" - recognize that carbon emissions aren't free: Steel is incredibly emissions intensive, and free emissions lead it to be used in areas where other materials would be better from an emissions perspective.
[1] https://en.wikipedia.org/wiki/Steelmaking#Carbon_dioxide_emi...
[2] https://www.cbc.ca/news/canada/sudbury/algoma-steel-420milli...