@avNYC --
Yes, the notice you received is "for real." In fact, it's been challenged in court, and the taxpayer lost. A non-resident sole proprietor who provides services to a CA-based customer is subject to CA income tax on that income, even if the services are performed entirely outside CA. See this reference for more detail:
https://www.coblentzlaw.com/california-office-of-tax-appeals-gives-precedential-authority-to-bindley...
Another reference: https://www.calcpa.org/news/2019/12/19/tax-net-widens
Your NY salary is not subject to CA tax.
You may take a credit on your NY tax return for the tax you pay to CA on the income that is taxed by both states, so you won't be double-taxed. If you've already filed your NY return, you'll have to file an amended return in order to claim the credit.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
