What should be done is a thorough analysis of what needs to be gone into in order to present a cohesive structure by which Sidereal characters and games are supported with as many tools as possible for the players.
Creation isn't a character. It's a setting in which to tell stories inside. And the Sidereal book should be all about Sidereals. One of those things should be how Sidereals engage and interact with Creation,because that is a major setting. The Sidereals book should also cover how Sidereals interact with Malfeas, the Wyld, etc, because there are devoted Conventions that cover important aspects from each of those locations and players need to have the tools that help them tell the stories they want to explore.
The primary goal is not "Make sure this group fits into Creation." It should be "Make sure this group is fun to play as."
The devs will have a set word count and they'll have to figure out what words are going where. And if the assumption holds that the Sidereals are still agents of the Bureau of Destiny, then it is a simple and unavoidable fact that the Bureau of Destiny needs to be covered by that word count. Otherwise, you're establishing an important character element without giving the players the tools to engage with it.
For information I would really appreciate about Creation from the Sidereals book? I want to know what each of the five directional Conventions do in Creation. I want to know what each of the Castes provide in their services to Creation as agents of Fate, and I want to know what the Bureau of Destiny does for Creation, along with its departments. After that? I'm pretty done with Creation, there.
What else would you want about Creation there? Because if it's not focused on Sidereals, there's a huge question as to why it is in the book that's about Sidereals.