You've given no citations that "US law" is somehow different than California law here. I've stuck to the specific citations directly applicable to their situation, but the idea that if the employer is the one who specifies the end date of employment, it's a firing not a resignation is pretty universal across the western world.
And yes, using "everyday language" to make public statements about a legal situation that if you used the legal definition you'd be making the opposite statement is very much a lie.
Additionally, even the "everyday language" argument has you making a distinction between "voluntary quit" and "voluntarily leave a job or other position". The crux is that if the employer specifies the end date, it's not voluntary on the part of the employee. Both under legal definitions and common parlance.
"Did Gebru voluntarily quit according to California law?"
"Is it obviously lying to say resigned?"
These are two separate questions.
Also I'm not convinced that it says anywhere in the California statutes that a letter of resignation requires an end date to be considered a letter of resignation. (It might say that about voluntary quitting, it might say that in case law, but I don't think it says clearly in any statute that to be considered a letter of resignation it requires an end date (which is different from whether or not it was a voluntary quit)