I would bet heavily on the accumulation. National average IQ has been going up by about 3 points per decade for quite a few decades, so there have definitely been times when Koko's score might have been above average. Now, I'm more inclined to say that this doesn't mean great things for the IQ test overall, but I put enough trust in it to say that it's not differences in intelligence that prevented the gorillas from reaching the prominence of humans. It might have slowed them down, but given this data it shouldn't have kept them pre-Stone-Age.

Given that the most unique aspect of humans relative to other species seems to be the use of language to pass down knowledge, I don't know what else it really could be. What other major things do we have going for us that other animals don't?