It occurs in the section called "Annihilation" (pp. 241-413), written by a Patricia Lowe Fox, described as a writer and journalist. It appears on pages 398-413, and there is a note saying that it was written by Vassili Grossman.
Here is the relevant section, on page 407:
So it appears I was mistaken in my recollection that in this specific report Grossman had said that the method of killing with exhaust gas was insufficent to have dealt with the vast numbers killed at Treblinka.The killing in the chamber took ten to twenty-five minutes. At first, when the new chambers were put to work, and the executioner could not regulate the gas production and was merely experimenting with the doses of the various poison gases, the victims were subjected to dreadful tortures, as they continued living for two to three hours. During the first days the suction equipment worked poorly, and then the tortures of the unfortunates lasted from eight to ten hours.
Various methods were used in killing. One was by inhalation of waste gases from the motors of the heavy tanks which supplied motive power for the Treblinka station. This waste gas contains two to three percent of carbon monoxide which binds the hemoglobin of the blood into one solid combination called carboxihemoglobin. This carboxihemoglobin is a much more durable conglutination than oxihemoglobin which is formed through contact, in the olveoli [sic!] of the lungs, of the blood with oxygen of the air. Within fifteen minutes the hemoglobin in the human blood combines with the carbon monoxide, and then the victim breathes into emptiness. Oxygen ceases to enter his body and symptoms of oxygen deficiency appear: the heart begins to work furiously and drives the blood into the lungs, but the blood poisoned by carbon monoxide fails to extract oxygen from the air. The breathing becomes raucous and a feeling of dreadful suffocation follows. Consciousness dims, and the victim perishes as if strangled.
The second Treblinka method, and the most widespread one, consisted of pumping all the air out of the chambers with large special pumps. By this method death ensued from approximately the same causes as from poisoning with carbon monoxide: man was deprived of oxygen.
And, finally, the third method, less widespread, was killing by steam, based also on deprivation of oxygen: the steam drove air out of the chamber. many kinds of poison gases were used too, but these were only for experiments. The industrial methods of mass killing were those mentioned above.
However, I do have a definite recollection of reading that claim somewhere. It may have been made by a survivor of Treblinka, or another commentator on Treblinka, or by Grossman himself in another context. The statement is connected with Grossman in my mind, since he claimed that as many as three million persons were killed at Treblinka (he makes that claim in the report quoted), and therefore may well have thought that such a huge number (vastly in excess of what is now believed) could not have been killed in a ten-month period by engine exhaust.
Another possibility is that the statement was made by Rachel Auerbach in her immediate post-war report "In the Fields of Treblinka", in which she certainly commented on Grossman's claim if three million killed, and criticised it. But I cannot be certain; I will have to check that work again when I get the chance.