I'm not sure if I'm feeling better or worse that it's a EU invention. Either way, it's hellof a corrupt practice.
ARENH looks like a mechanism by which France wanted to entice competition in end customer sales (and distribution?) of electricity.
https://fsr.eui.eu/regulated-access-to-incumbent-nuclear-ele...
France chose to use the mechanism of ARENH. This isn't an EU thing.
That's usually how that works. The EU makes rules and national parliaments create local laws to comply.
Of course they could have chosen another way to comply, for example breaking up EDF. But they didn't want to do that, probably for good reasons.