I'm quite uncertain about this project, in part because there are so many ways in which a book like this could be written, produced, and shared. However, while I have no prior knowledge of de Haan's work, their MIRI grant and presentation of the proposal passes my gut check for a high-variance creative project like a book.
I appreciate that the author is focused on longtermism and sentientism and that they have an interest in FDT. I wouldn't be too excited about a more generic pitch for such a wide-ranging book, but I think with particular angles like these, it could be more unique and promising. On the other hand, I expect a successful book to usually have a clearer vision and pitch for that uniqueness at the current stage of this project.
I don't have a good sense of what sort of publication is being targeted, and my prior for the success of a self-published book without a significant author platform or exceptionally creative marketing strategy is quite low. I also think two years (even one year) is a long time to write a successful book like this, at least unless there is a lot of original research (e.g., travel, interviews).
I'd be more excited about a smaller grant to develop the idea further and explore alternative strategies like a highly targeted and themed blog, newsletter, forum post sequence, or social media presence. I see my tiny impact certificate offer primarily as a skin-in-the-game upvote for this direction.