It's remarkable how you can speak with such self-asserted authority about what's in the report without referring to the actual words of the report:
Between now and 2020, there are no technological constraints to accelerated deployment of the major renewable resources with existing technologies.
Sure, increasing renewables requires upgrading our aged, dilapidated power grid. I'm not sure what purported laws of nature this goes against. Somehow the NAS report - written by scientists with known credentials, unlike anyone posting in an online forum - seems to say the variability issue is anything but insurmountable:
Balancing wind with multiple renewable resourcesincluding solar, which does not normally peak when wind does, and baseload power from geothermal and biomasscould mitigate the temporal variability in generation. Reaching the goal of 20 percent nonhydropower renewables by 2035 could be achieved by adding 9.5 GW per year of wind power and a total of 70 GW of solar PV and 13 GW each of geothermal and biomass. Using multiple renewable resources to reach this level would take advantage of the geographical variability in the resource base.
It's true that they didn't spell out in detail scenarios that go beyond 20%, but that scarcely implies they regard that as an absolute limit but reflects the assessments available. The same NAS report also talks about a 12-20% increase in nuclear by 2020. Does that mean they think that's the most nuclear could supply?
To quote a "real scientist (TM):"
I'm sorry you don't like what the scientists are saying; but neither do the climate deniers.