I think when people talk about "exodus" form California, they mean domestic migration mostly. Imagine the most extreme scenario (completely ridiculous in reality, but let's consider it for a moment) - all current California population has moved out, and then has been replaced with fresh immigrants. Would you consider this situation as "population exodus happened in California", in the same meaning as it is discussed now, or it would be more appropriate to say "nothing special happened"? I think it would be the former rather than the latter.