Failure By Analogy
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We definitely understand how neurons in several areas of the brain (e.g. retina, auditory cortex) do their information processing. Simulations of these areas did produce expected results. So there is reason to believe that simulating a whole human brain with high enough fidelity would lead to similar results - i.e. intelligence.
A simulated bird with high enough fidelity to physical reality will fly correctly in a simulated environment. The plane/bird argument does not work for precise enough models (physical or simulated).
Now, you could claim that a precise simulation would be overkill in terms of required compute power. That could definitely be the case. However, we will probably have enough readout and simulation capability for a human brain in 15-20 years. A suitable "airplane" (de-novo) design has that timeframe to succeed before a brute force simulation will get there first.