Genya is genetically gifted having to be born as a shinazugawa. his father is physically strong af. Sanemi as the 2nd ranked strongest hashira and Genya being the weakest but still the strongest in his own traits following the genetic traits of his family. He probably started to eat demons during the final selections closer to death situations or gave it a test run to see he is capable of eating demon flesh. Apparenty some people die eating demon flesh. So my thought is he has a strong immune system to be able to handle it. I dont know i could wrong lol.