From the author's introductory notes:
Many of the accounts of what has happened in the Trump White House are in conflict with one another; many, in Trumpian fashion, are baldly untrue. Those conflicts, and that looseness with the truth, if not with reality itself, are an elemental thread of the book. Sometimes I have let the players offer their versions, in turn allowing the reader to judge them. In other instances I have, through a consistency in accounts and through sources I have come to trust, settled on a version of events I believe to be true.
In other words, he has no idea whether it's true or not.
Amazing it, and this a-hole has the gall to say this book will bring down the Trump presidency? Isn’t that sedition? Sounds like it to me!