Koko's language skills were misrepresented in a pretty extreme way. Very few recordings of Koko communicating anything more than short phrases were ever released, and those that were never demonstrated much more than word association. I've met dogs and pigs that were able to go fetch the "carrot" versus the "bunny" stuffed animals based on verbal commands, but I'm not going to claim that constitutes mastery of language.
Koko primarily communicated via repeating individual words that she'd been able to correlate with getting immediate effects. "Food", "Water", etc. When she did use multi-word phrases, they had no semblance of grammar: human babies will quickly work out the difference between "Me bite him" and "him bite me", whereas Koko never did. Frankly, despite being billed as a decisive blow against the Chomsky school of linguistics, Koko probably stands as strong evidence for it.
IMO, the massive complexity gap between human-level language compared to the relatively tight pack of merely possessing vocabulary animals, pretty directly correlated with levels of problem solving skills and tool use, should signal that there's something to the idea that our cognitive and language abilities are pretty closely linked. Mimicking human behavior may not be necessary for intelligence, but some kind of language likely is.