I remember the story of the researcher who had an extended conversation with Koko for a while before he remembered she wasn't human. He said it was a profound feeling to speak to some one else in his native language. The first language he ever knew -- he's hearing but both his parents are deaf.
And to have someone not be another human but an ape - again very profound emotions.
She knows what she's saying. The cuts are suspect and man's interpretation and manipulation is suspect, but Koko herself is real and her ability to think and communicate is real.