They mention that the report found that the "movement patterns for wealthy areas were similar to those in less wealthy areas". This is kind of vague and I don't know what they mean by "wealthy areas". It would be interesting to see if they looked at homeowners versus renters who left wealthy areas. I didn't look at the study itself, so maybe they did that kind of analysis.
The reason this is important, because even a small percentage of really wealthy people leaving the state, would cause a large dip in taxes collected. So even if it turns out 1% of the wealthiest Californians left the state, that could cause big deficits in the state budget. Even if there is a bailout by the feds for this year, it could cause a lot of problems going forward.