Okay, looks like Turbo Tax is right, no penalty is owed.
https://www.ftb.ca.gov/forms/2018/18-5805-instructions.html
Those are the TY2018 instructions -- couldn't find an equivalent link for TY2019 instructions, but I assume the language is similar: "If in the prior year your tax liability, less any credits for the prior year, was less than $500 ($250 for married/RDP filing separately) you are not subject to the underpayment of estimated tax penalty."
And the TY2019 5805 even says it (but it wasn't clear to me at first): "... for either 2018 or 2019 was less ..."
So CA is actually more lenient about penalties than the Feds in this circumstance -- go figure.
Anyway, sorry for the noise, mystery solved.
