Webhe's not a demon. It's explained very briefly in the manga, but genya happens to have a unique ability to gain strength from eating demon flesh (or things that come from. Webgenya having strong digestive system is real. It was stated in databook. I remember reading it in manga as well. Plus there are some section of people having rare blood. Webin chapter 136 genya says gyomei figured it out and even told genya to go to shinobu so she could examine him and make sure his body wasn't negatively affected.
