Quite interesting. One limitation is that the data was collected and analyzed on a county level, which skews the analysis towards California. In California there is combination of large scale food production combined with much larger counties. Thus major California agricultural counties will inevitably show up as the largest.
The U.S. Census Bureau tracks food shipments based on dollar value. Because California's specialty crops (citrus, avocados, fresh berries, almonds, walnuts, pistachios, for example) are worth so much more per lb than grains, for example, the data will skew heavily in California's direction. If it were measured by weight, there's no doubt that the Midwest corn and soybean states would skew much heavier (pun intended). On the other hand, a large proportion of the Midwest grains are used for animal feed, so I'm not sure how that's accounted for in the study cited.
Yes. California - as with most of the western US where there are major irrigation water projects - grows high-value specialty crops. Also the growing season is longer in California, which often allows for double or triple cropping. (A tomato grower in Stanislaus County will get two, and sometimes three, crops per year.)