First of all, the "professional activist" gig that you're talking about is one with a power-law distribution. Most people who speak out against bad behavior by employers end up losing for it. (I can speak to this one, personally.) Sure, there are a few people who hit big and make enough money to support themselves, but they're very rare.
From an expected-value perspective, she reduced her earning potential, even including the possibility of winning the suit. While she was asking for $16 million, as a lawyer, she probably knew that even had she won, she'd be lucky to get 1/5 of that, net of attorney fees. Meanwhile, she endangered her ability to ever return to venture capital.
Also, Sheryl Sandberg isn't really a professional activist. She's establishment. "Lean in", as I understand it, isn't about up-ending the corporate regime but adapting to it.