I think it's a question of semantics and definitions. A taxpayer who changes his state of domicile in 2021 becomes a part-year resident of both states for that year. He does not become a non-resident of CA - unless he has income from CA sources after the move.
For tax purposes, a non-resident is a person who is domiciled in one state, but who has income from another, non-resident state. Example: a resident of Oregon who works across the border in CA. Such a person files taxes as a non-resident of CA and a resident of OR.
This reference has a detailed explanation of California's residency rules:
https://www.ftb.ca.gov/forms/2021/2021-1031-publication.pdf
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
