What if the language weren’t so direct? For example,
> If I can’t know who is providing reviews to my work, it’s difficult for me to imagine continuing to work as a part of this group.
Many people would still describe this (erroneously) as an ultimatum even though it could be bleakly summarized in the same vein as a true ultimatum.
It's more nicely worded to be sure, and definitely less aggressive, but it still says the same thing, which is "I'm gonna be an unhappy employee if I can't find out who reviewed my work". If they aren't willing to tell her who reviewed her work (which they may or may not have valid reasons for doing, but clearly they don't want to do), then they are dealing with someone who is going to be an unhappy employee since their conditions won't be met. Sure, not a resignation, but if your employer doesn't think you'll be a happy employee, why keep you around?
In any case, she has been very vocal on twitter and has not seemed to deny that she gave some sort of ultimatum. If she didn't give an ultimatum, it would only make Google look worse, so why not mention that on Twitter (given that she has tweeted probably 100 things about this incident in the last two days)? Given the absence of a denial, I'm going to assume that it was worded as an ultimatum.
The thing is, there's a big difference between resigning and being fired for cause, even if both end with you not working at the company anymore.