I agree that neural nets in general, and the human brain in particular can't be readily replaced with a well-structured computer program of moderate complexity.

But I indeed was presuming computationalism, in the sense that "all a human brain does is compute some function that could in principle be formalized, given enough information about the particular brain in question". If that's the claim you wanted to focus on, you should have raised it more directly.

Computationalism is quite separate from whether there is a simple formalism for intelligence. I believe computationalism because I believe that it would be possible to build an accurate neuron-level simulator of a brain. Such a simulator could be evaluated using any turing-equivalent computer. But the resulting function would be very messy and lack a simple hierarchical structure.

Which part of this are we disagreeing on? Do you think a neuron-level brain simulation could produce intelligent behavior similar in character to a human being? Do you think an engineered software artifact could ever do the same?