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IANAL, but my best guess is that she was just let go without cause. Your employer can fire you at any time and technically doesn't need a reason. Typically, "terminated with cause" is a specific thing where they fire you and give a specific reason (e.g. stealing) that might have bearing on whether you receive unemployment benefits, accrued vacation, etc. It's hard to imagine that they fired her in that way and that she was just plain-old-fired (there's a reason for it, but not legal cause).

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.