Electric arc furnaces don't necessarily reduce union labor needs. They reduce the amount of energy needed and reduce the carbon emitted and are simpler to operate (yes, fewer people). But the newfound flexibility and utilization that they provide generally increases the number of union laborers the steel mill employs.

The IBEW brings union workers in from multiple states and puts them up in temporary housing just to build these things as fast as possible. Everybody wants them, they are just so incredibly expensive at the size that US Steel needs.