In 2018 California had 954,000 businesses[1] according to the Census bureau. If we take the "13,000 have left between 2009 and 2016" (why stop there?) that would represent nearly 1.4%. However the Hoover Institute doesn't actually tell you if they were "small business" or "regular businesses".
That can be important because there are nearly 4 million small businesses in the state[2] and a departure is a departure, small, medium, or large. If use 3.9 million as the total number of businesses that is just .3%.
Every year people argue "California taxes too much, businesses are leaving in droves" but that isn't supported by actual business license data nor employment data. Over the last 10 years California consistently has the largest economy of all 50 states.
Nobody likes taxes, and I personally don't like misleading articles.
[1] https://www.census.gov/quickfacts/CA
[2] https://www.sba.gov/sites/default/files/advocacy/2018-Small-...