whistleblowers reveal criminal/corrupt activities to gov't officials for possible prosecution.
She wrote a book ostensibly about her time at Meta, she also signed a contract agreeing to not disparage her former employer.
Presumably the agreement doesn't prevent her from talking to the gov't as a whistleblower/witness, but it likely clearly prevents her from publishing a disparaging book about Meta, promoting her disparaging book, or talking to the press disparagingly about Meta.
Her lawyer is fighting for her right to publish/promote/sell her disparaging book despite the contract she (presumably) signed freely. (If she was coerced/forced, there are likely laws about that, but that's not what the lawyer is arguing.)