I agree with him, in real life, we are not able to combine spiritual energy and physical energy to form chakra, due to lack of chakra system...
But, let me clarify one thing, in NU a person without chakra means both spiritual energy and physical energy are depleted totally, so life force is not able to remain within a useless body, as its cells lack physical strength and mind lacks spiritual strength.
It might be the case that ninjas utilise chakra in everything, so it is very difficult for their body to work without chakra (as it becomes their second nature) i.e chakra always remains flowing even in their chakra network involuntarily even in minimal amount which is the basis of genjutsu in NU.
