This not a “law”. Here’s what EU spokesman said:

From your link:

The bizarre diktat from Brussels has far-reaching implications for member states, including Britain, as no water sold in the EU can now claim to protect against dehydration.

Sounds like a law to me.

Also,

Moreover, this advise ruling was not an initiative of EFSA, so it’s not a case of a government having too much time on its hands.

It took them 3 years to investigate and make a ruling that water won’t protect against dehydration. I would disagree that this isn’t about too much time on their hands. It should have taken someone 20 seconds to state the obvious, of course drinking water can help protect against dehydration.