This is penance and damage limitation.
I've no doubt that Bloomsbury were hammered after the JKR Twitter pile-on, and were lobbied hard by the activists to balance things out, in lieu of actually cancelling JKR.
They know that in terms of publishing (if not in terms of personal cost) JK is untouchable - no publisher is going to cancel her, and they knew that, so the next best thing would be for Bloomsbury to throw money at a book to (in their eyes) provide fair balance.
Hence the six figure sum, the opportunity to discover all the ways that JK and women are wrong, to create a permanent print narrative about the history of 'gendered souls' (or however MB frames it) and the history concept is to presumably cement the fact that gendered souls have been here forever, by enveloping them in a certain amount of simplistic social history about women.
Creating a duel narrative is presumably supposed to give it credibility. What worries me greatly though, is the reference to adolescence - this is possibly going to be the massive push to affirm kids - 'don't let them suffer like I did'. The framework will be designed to persuade everybody that there have always been hundreds of thousands of trans kids and this book will be the manifesto to 'educate' everyone.