There's a PDF (https://prosperitynow.org/sites/default/files/resources/Bank...) that says that an analysis with those figures was done by Matt Fellowes at the Brookings Institute, which is interesting because it doesn't mention any polling or details, so who knows how those numbers were generated.
Looking at the FDIC study from 2009, ~20% of Black/Hispanic households in California were unbanked, and when looking at citizens the number is ~7%.