It seems some people here didn't get a few things right:
1. Muzan injected pretty much all of his blood into Tanjiro( although by doing so, he "logically" yeeted that 1000-effect cure into him but oh well, plot armor? ) before dropping dead, although it's utterly stupid how he suddenly became...
I guess Kukoshibou was completely right about killing "demon-eaters" by cutting them in half. But with Genya, the split had to be a vertical one.
Still somewhat shocking since he ingested cells of an UM1...