I think you are arguing that this is more straightforward than it really is. If we suppose (in absence of her actual email) that she declared "...make x, y, and z happen or I resign", that certainly reads as a (conditional) offer of resignation that can be accepted by the other party.
All of this is likely to end up in court and we'll get a decision on whether it was a resignation or firing.
"However, we believe the end of your employment should happen faster than your email reflects because certain aspects of the email you sent last night to non-management employees in the brain group reflect behavior that is inconsistent with the expectations of a Google manager."
That isn't accepting previously offered conditions. I agree what it means legally will probably end up in court.
[1] https://twitter.com/timnitGebru/status/1334364735446331392
In the United States, there is a huge difference legally between being fired and resigning. You don't get to declare someone else's "resignation" for them.
Maybe that's the logical thing, but lots of people also bluff and fail.