[yqx]

That's because that's only half of Hill's argument. If you read on you'll see the main point is about the size of the largest groupings, which in humans appears larger than what has been found in Gorilla's to date.

> “[T]he extreme social brain hypothesis doesn’t claim other primates don’t form hierarchically increasing groupings,” Hill wrote in an email to Science. “It focuses on the size of the largest human groupings.” Humans recognize and remember details about more than 1000 individuals, he notes, whereas the “highest level groups in the gorillas are not even as big as large chimp communities.”