This claim is rooted in some theoretical considerations that don't obviously rely on directly sampling things.

In that case, can you expand on what those theoretical considerations are? As far as I can tell, the main thing that allows the launch of civilization is speech. But even people well below average intelligence can speak. Moreover, most of the major advances seem to be made not by the average individual but by the outliers. The idea that the species as a whole needs to be as intelligent as we are to make a civilization seems problematic. There may be issues here with what one means by civilization and how one is measuring intelligence.