Ceres' reasoning was quite simple; she didn't entirely trust the Lie Detector in this case, but if there was even a slight, remote chance that Victor wasn't also from Garden like she was, then he had to be from somewhere, obviously.
Calling him an Irregular might have been a stretch, maybe... Those guys were Singularities among the Awakened, and only a handful of them had ever appeared throughout history. Common sense or standards did not apply to them, and their very nature and existence made a point to constantly rebrand and redefine their fates and destinies as Awakeneds.
Ceres would know; she had extensive knowledge about Singularities and Awakeneds classified as Irregulars implanted in her from a very tender age, after all. That was the entire point of her existence.
