In the Afterword, Steiner explains that in order "To reconstruct the history of Treblinka we have relied almost solely upon the testimony of the survivors." 
In the Preface, no less an authority than feminist icon Simone de Beauvoir vouches for the veracity of the content: "Each detail is substantiated by the written and oral testimony he has collected and compared." 
In the Introduction, Terrence Des Pres states that "Treblinka is as close to the facts as we are likely to come." 
Limited concessions Steiner owes to having made to fictional reconstructions include the dialogue between the prisoners; done, he said, to give a storyline momentum and sense of immediacy to the action; pseudonyms for the actual names of Treblinka survivors; and reconstruction of several scenes -- but always, to be sure, as was noted above, based on eyewitness testimony he gathered from Treblinka survivors.
Verbs like "relied...upon" and "substantiated," and phrases such as "close to the facts" and "reconstruct the history," all project an honest effort to reach for factual truth. But as Steiner informed OSI attorney, Betty Shave, the most important part of his book actually consisted of make-believe fantasies.
OSI stands for Office of Special Investigations, the Nazi-hunting arm of the US Justice Department. The reason Steiner was corresponding with OSI lawyers back in 1984 was because he insisted he had written a factually accurate account of the Treblinka uprising; coming, to borrow the words of Terrence Des Pres, "as close to the facts as we are likely to come."
OSI lawyers were then seeking to extradite the retired Cleveland autoworker, John Demjanjuk, to Israel -- claiming Demjanjuk was a sadistic Treblinka guard going by the moniker of "Ivan the Terrible" -- and they were keenly interested in this pivotal scene in Steiner's book, describing Ivan's apparent murder at the hands of a Jewish prisoner:
Adolf is running toward the gas chambers. He is going to set fire to them. Suddenly Ivan, the sadistic giant, appears in his path. The Ukrainian seems a little bewildered, surprised, but not frightened. His black eyes stare at Adolf, Adolf's hands, Adolf's belt, looking for a possible weapon. They do not see one. Ivan decides not to draw his revolver. His knees slightly flexed, his hands open, he waits for the little Jew who keeps running toward him. Ivan smiles. He is completely at ease in his skin, in his body rich with blood,flesh and muscle. He blocks without flinching when Adolf tries to butt him in the stomach. Knotting both hands around Adolf's throat, crushing him with his full weight, he begins to strangle him. He dies in the act. One minute later, when Djielo reaches his friend's body, he will see first the the wide back of the Ukrainian, and then the dagger planted in it with Adolf's hand still clutching the handle. Adolf's dead body is covered by Ivan's, but in his eyes is an expression not usually found on the faces of strangled men. It is as if, at the very moment he died, Adolf felt only the immense joy of knowing that he had finally managed to unsheathe the Ukrainian's dagger and had dealt him a mortal wound. 
Steiner assured OSI lawyers that the "story of his [Ivan's] death, during the insurrection, was completely imaginary."  Likewise almost completely imaginary were the details surrounding his portrayal of the August 1943 Jewish prisoners' uprising:
"Being unable, in the course of my investigation, to gather but very few accounts of the final insurrection [Steiner wrote Betty Shave], I found myself ... restricted to imagining the details of its unfolding."
"The death of Ivan," he reiterated, "is purely the product of my imagination," explaining that he had been driven to lie by wishful thinking. Later on, he would blame co-author, Gilles Perrault, for leaning on him to crank up the melodrama in the book's climactic scenes.  What he and others insisted in the book's Introduction, Preface, and Afterword was a scholarly chronicle of certain tragic events had, in fact, been a melodramatic novel. When he wrote the most important part of his book, describing the August 2nd uprising, including a portrayal of the death of Ivan ("the Terrible"), Steiner had finally to confess that he relied almost solely on his own imagination.
Interviewed in 1986 by the French newspaper Journal du Dimanche, Steiner said that his most reliable informant for all that happened at Treblinka had been a Holocaust survivor named Eliyahu Rosenberg. 
In the interview, he noted that Rosenberg had sworn an affidavit saying Ivan was beaten to death with shovels as he slept on his bunk bed (not knifed to death standing up in an open place as described in Steiner's book). He added that if Rosenberg claimed Ivan was killed, then there was a 99% likelihood that Ivan was dead.
The following year, at the John Demjanjuk trial in Jerusalem, Rosenberg, a witness for the prosecution, would identify Demjanjuk as having been "Ivan the Terrible" of Treblinka, thereby flatly contradicting his own sworn affidavit of 1945 and his sworn affidavit of 1947.
An article re-evaluating Steiner's book Treblinka recently appeared in the French journal Revue d'histoire de la Shoah. There it was made abundantly plain Treblinka was nothing less than a deliberate attempt to hoodwink the reader into believing the book was a real work of history .
Didier Daeninckx: "...ce faux roman qui fut présenté comme un vrai document sur la réalité quotidienne d'un camp d'extermination."  Translation: "...this false novel which was presented as a real document [chronicling] the daily reality of an extermination camp."
Two words used by the very resolutely anti-revisionist historian Pierre Vidal-Naquet to denounce Steiner's imposture include "fabrication" (in French: "falsification") and "forgery" ("contrefaçon").  Vidal-Naquet, obviously angry at having been duped, also uncharitably qualified Steiner's book as "un piège tendu," meaning a booby-trap.
Other readers, including many formerly sympathetic and supportive scholars and journalists now, of course, complain bitterly of having been intentionally deceived by Steiner and his novel-as-true-history on the Treblinka concentration camp.
Shades of Binjamin Wilkomirski and his allegedly, till recently very much ballyhooed, genuine book of "memoirs," Fragments: Memories of a Wartime Childhood!
1. Jean-François Steiner, Treblinka, Mentor: New York, 1979, p. 303.