They're not arguing it's an economic harm to decarbonize, they're pointing out that it cannot physically be done, at all, without some way of storing energy.

This is needed because otherwise you'd only get energy when the wind is blowing and the sun is shining. This is especially true if you do not classify nuclear energy as green energy, but even with nuclear providing baseload generation you would want energy storage to truly take advantage of the energy that renewable sources can provide.