Speculation? Because she was good friends with Gebru, and was continuing the (internal) drama that they wanted stopped. Assuming that Jeff Dean actually took the right decision in firing Gebru (I tend to believe so) - Google really can't blink here; they'd be at the mercy of vocal employees if they did. It's basically a cancer-like situation, you either cut it all & apply radiotherapy (however unpleasant), or it will overwhelm & kill you.
https://www.ncsl.org/research/labor-and-employment/at-will-e...
"At-will means that an employer can terminate an employee at any time for any reason, except an illegal one, or for no reason without incurring legal liability. Likewise, an employee is free to leave a job at any time for any or no reason with no adverse legal consequences.
At-will also means that an employer can change the terms of the employment relationship with no notice and no consequences. For example, an employer can alter wages, terminate benefits, or reduce paid time off. In its unadulterated form, the U.S. at-will rule leaves employees vulnerable to arbitrary and sudden dismissal, a limited or on-call work schedule depending on the employer’s needs, and unannounced cuts in pay and benefits."
Welcome to the US :D
It is possible that there are many employees who are committing "fire-able offenses" at many companies but because the companies are not trying to get rid of these employees, these offenses go undetected or no serious action is taken if they are discovered. That's life.
The question here is whether Google wanted this employee gone, and if so, why. It sounds like she was reading documents associated with the other employee on her team who was fired. We are not told of any actual damage that was caused.
The quoted statement in the top comment states "After conducting a review of this manager's conduct...". They have discretion over what they choose to do after the review. They wanted this employee gone. Why?