ΔE 17.7, loose: different enough that you should pick on purpose. Magnesium is noticeably lighter than Iron Hands Steel.
Can I use Magnesium instead of Iron Hands Steel?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Magnesium is noticeably lighter than Iron Hands Steel.
Can I use Iron Hands Steel instead of Magnesium?
The same difference applies the other way round: Iron Hands Steel is noticeably darker.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.