I'm just going by the CNN article and her tweets, which include the response from some manager, Megan:

As a result, we are accepting your resignation immediately, effective today.

https://twitter.com/timnitGebru/status/1334364732480958467

Looks like Timnit Gebru made an ultimatum and offered her resignation if the terms of the ultimatum were not met. Google accepted her resignation and chose to have it effective immediately (as is often the case when you resign and a company feels like it would not be beneficial to have a longer departure period).

It's not like I'm taking only the words of others. I'm reading what Gebru herself chose to put out there.