Earlier quoted context omitted.
Yes, that's not a resignation. At google, to technically resign involves filling out a form on a website, it's not something you can verbally state to your (manager's) manager. Again, Google can still say this and probably even thinks internally that's what happened, but having resigned twice from Google, that's not how it really works.
I'm pretty sure if you say to your manager "I quit" and stop showing up to work, regardless of what internet forms you fill out, it's a resignation. The fact that Gebru gave her resignation in the form of an ultimatum rather than whatever website form you're talking about is a pedantic point. "I resign if you don't do X." "We're not doing X, we accept your resignation." This is a resignation the way I understand it.
Let’s think of a hypothetical. A student says “I’m going to quit eventually in order to attend classes” and the manager says “I accept your resignation, you are fired”. This generally seems more like a firing because somebody was going to resign, more than it seems like a resignation no?
In the Gebru case, Gebru didn’t express WHEN she was going to resign, and that’s google’s issue. Maybe Gebru was, for all Google knew, threatening to quit in a month or a year or in 10 years. Gebru never actually made the time of her proposed resignation clear. As such, the law doesn’t consider to actually have made a legally binding resignation.