Why did neither Ephrem nor Aphrahat know the Diatessaron of Tatian?

According to their own words, Ephrem and Aphrahat annotated *The Gospel*, not a text called *Diatessaron*. How is it possible that these theologians had never heard of this work, let alone of its supposed author while their contemporary Eusebius had all this information at his fingertips? Did our blasphemous heretic covertly remove his name and replace the title with «The Gospel» before spreading his mischievous text among the unsuspecting Syrians? Is that what we are to believe?