The fact that a non-resident independent contractor who provides services to CA businesses must pay CA taxes on that income is a very recent development, and is due to two court decisions made this year (2019) - the most recent of which was a month ago.
Thus it's very possible that you may run across older answers on this board that were correct at the time, but are correct no longer.
You can read the details of those court decisions here: https://www.coblentzlaw.com/california-office-of-tax-appeals-gives-precedential-authority-to-bindley...
In May 2019, the California Office of Tax Appeals ruled very recently that income from California-based clients is taxable as California source income.
For some discussion:
https://www.coblentzlaw.com/california-office-of-tax-appeals-gives-precedential-authority-to-bindley...
https://money.stackexchange.com/questions/95243/is-self-employed-work-performed-out-of-state-for-a-c...
