Their math seems off. I really don't see how you get to 97% renewable when the state was burning natural gas to generate 2.8 GW of non-renewable power and had about 1.1 GW of nuclear at the time. There might be some funny math here where they assign all of the "dirty" power generated in the state to exports and can claim that that California is running on renewable power.
Source: https://www.caiso.com/TodaysOutlook/Pages/supply.html