Individualism (like reductionism more generally) only requires that the higher level explanations be expressible in terms of lower level explanations, not that they're necessarily better (in terms of the insight per unit complexity) for any particular context[1]. It certainly doesn't require any analogy between the people in a nation and the neurons in a brain.

[1] like how an atom-by-atom model of an airplane might be better, even if it would be accurate enough to break it into a smaller number of tension- and shear-bearing elements