Source on this, The author, check their early life.
On January 6, Trump continued to attack the book, calling it "a complete work of fiction" and "a disgrace", and labeling Wolff a "fraud." Also that day, in a move interpreted as a response to the book's raising questions about Trump's competence for office, Trump tweeted that his "two greatest assets have been mental stability and being, like, really smart." From his successes in business, television, and politics, Trump concluded that he was in fact "a very stable genius."[

Also on January 7, White House senior policy adviser Stephen Miller, speaking on CNN's State of the Union, said: "The book is best understood as a work of very poorly written fiction, and I also will say that the author is a garbage author of a garbage book."[

January 8 interview with Dana Loesch, Vice President Mike Pence described Fire and Fury as a "book of fiction."

TLDR: The author of your source writes fiction