Events play out on different days. For example, the book places Thor's coronation as before Tony's party (I assume it's the same day) while the comic/old timeline has it the following day. The book also has Betty finding Bruce being around the same time as the Stark Expo battle and Thor trying to retrieve his hammer on the SHIELD base while the comic/old timeline has the Expo being the night before the two other events.
I know the difference between the two, I just don't think Marvel Studios considered it when mapping out the week for both What-If and now the book. If the comic was still considered as canon as you believe, I don't see why it wouldn't have been matched. Whether people think they're canon or not, I think it's clear Marvel Studios doesn't follow the history in them the same way they do their movies/shows, and contradictions aren't too surprising.