This is fan fiction. The reason it matters is as a proof point that the farrago of EU regulation, labor markets, supply chains, trade policy, … is adequate to support battery production at scale.

You walk in, as the EU, and assume control of the facility, by force if needed. The value is that the capacity exists within the bounds of your nation state control.

China knows this, developed countries that lost their manufacturing capacity are relearning this.

> if South Korea stops all cooperation with the EU tomorrow

That doesn't happen between democracies and hasn't for generations, except for one democracy recently. I don't know that it happens between any significant economies, outside of wars (when and where has it happened?), except one recently. Trade is reliable, despite the nationalist attempt to use FUD. That's how countries get access to the best products and sell their best products.

If it does go both ways (say "EU stops all cooperation") and the effects are the same, and no one wants the factory to actually shut down, does something start to matter more/less then?