People have argued both cases. The tl;dr is that she said "I will resign if XYZ" and Google was like "we accept your resignation effective immediately." Whether or not that counts as firing is a matter of opinion.

well; also whether google has to pay severance.

There’s no legal requirement to pay severance. That’s just a courtesy most companies do in exchange for a non-disparagement agreement or during layoffs to save face a little.

She made demands and gave a resignation date if those demands were not met. Google said that they could not meet those demands and accepted her resignation but because of her conduct (emailing hundreds of Googlers), the resignation date was made immediate.

I believe on currently available information she did not give a date, but said she wanted to "discuss her exit timeline".

Also it's likely that her contract allows Google to provide her pay in lieu of her working out the end of her contract, so they are probably able in general to move dates like this (not commenting on the other aspects of this situation though).

Timnit: "if you don't give me x, then i am going to do y in a week."

Google: "we aren't going to give you x, so you can have y. in fact, you can have y starting right now."

i'd say she tendered a conditional resignation and they called her bluff and accepted it.

It's fuzzy, and people are disagreeing about which aspect to emphasize.