The harsh reality is that it genuinely doesn't matter what the UN declares. General Assembly resolutions are non-binding and are almost universally ignored.
The UN also is not the ultimate authority on international law. The countries subscribing to said international law are.
In this case the claim of illegality is backed up by a court the UK was one of the original proponents of, that the UK has presided over, that the UK was the first state to submit a case to. As such, whether or not the decision is enforceable it's quite a bit more than "just claiming it's illegal".
The countries subscribing to said international law voted 116 to 6.
Unless the 116 countries voting to uphold it are actually willing to do something about it, it's near completely meaningless.
Can we stop pretending that the UN has power over anything?
Is any action enough to make it meaningful? Do they have to start a war? Win the war?