The entire study of great apes and sign language has been based on flawed methodology and subjective and biased interpretation of very small data sets.

Its interesting that apes can recollect abstract symbols. It's even kind of interesting that they can to some extent recollect hand gestures. But it is nothing more than symbolic association at its absolute best. Calling it language is a fundamental misrepresentation of what is taking place. Apes already possess several kinds of 'language' comparable to symbolic association, stuff like emotive language and body language and expressive language. There is no substantive evidence that they are capable of understanding and using an abstract language.

What has largely happened in so called 'studies' on 'sign language' in great apes, has been a lot of animal abuse and fundraising for animal abuse predicated on vague notions of how inspiring the idea of talking apes is. They can't talk. They are nonetheless very interesting creatures and we should be fascinated by them even without them having the ability to speak human language.

The really frustrating part is that they shouldn't have to speak with us for us to feel compassion towards them. The really disgusting part is that wild animals were being abducted from the wild and raised in deplorable conditions while essentially being tormented by disgraced researchers trying to prove that they could talk. They're very well suited to their natural environment (which we are destroying) and are not meant to live lives in concrete cages on the other side of the world being prodded and clicker trained to make vague hand motions. It's just animal cruelty under the guise of scientific research.