In a sense, defining away human intelligence is the whole point. We have no servicable definition of intelligence other than a behaviourist black box embodied by humans. From the perpective of AI-research any progress will chip away bits from the intelligence black-box and still the resulting AI will be out of reach of "true" intelligence. Since we have little knowledge of the internal structure of "intelligence" as a phenomenon, we can't tell how much is left to discover about it.
From the other end, the merits of production AI will be measured against isolated properties pulled out of the intellgence black-box, and since intelligence remains undefined (in a strict ontological sense), debate ensues.