For what it's worth the only reason the bomb was a threat at all was because they mixed in poison coated caltrops and such that were slowing his regen
But the Rasenshuriken kills cells which ironically means it would absolutely be able to kill Muzan even if we ignore the power disparity since the reason for his regen and ability to survive without a head is because his cells have his thoughts/memories inside them
You can absolutely kill demons, even Muzan, as long as you can get every cell in their body at the same time.
What are you talking about Cross? Don't you know Demons can theoretically regen from nothing? ONLY Sunlight can kill Demons, not Nine Lives, not Excalibur, nothing but Sunlight and Nichirin Blades.