Again, you're tackling this from the frame of mind of being certain that Google would win. It's not about the money: $90 million is almost certainly cheaper than what this case would have cost. It's about the reputational damage: Rubin potentially winning a settlement against Google would have been immensely embarrassing.
It's all about doing what's in the best interest of the alleged victim. She would have probably had to testify at trial. And imagine the hell it would have been to have a settlement paid out to your alleged abuser, thereby implying that you're a false accuser. Juries can be unpredictable, its easy to see why Google decided to find acceptable terma to part with Rubin.