Apparently Genya has some unique demon-consuming abilities that lets him develop a Blood Demort Art from eating the flesh of Upper Moons, while maintaining his humanity. I’m not sure what would happen if any other slayer consumed a high-level demon’s flesh, as that’s a valid way of being exposed and turned (Genya on the other hand stays a person.)
If anyone’s making demons, willingly consuming their flesh to become one can work well for their backstories. (As opposed to the usual demon coming around and transforming them. “Hey, check this out. I just bought some demon flesh off the black market, it’s supposed to give you insane powers and make you immortal, but don’t tell the authorities.” This opens a potential arc for the protagonists to dismantle an illegal ring that trades in demon meat, much like the Mystery Eater circulates human meat.)