(This turned into an essay sorry!!) But most of them didn't even actually know ASL so I assume they were unaware of simpler form of ASL or any other signed language.
Additionally, while I'm not a scientist of any kind, based on what I've seen on posts about this and in the video, while non-human animals like apes and dogs or cats or lizards even can learn to associate specific sounds with an action or itemâlike your dog knowing when you say outside or walk they get to go outside or on a walk, or that ball means you have a ball to throw for them, or that lady on tiktok who trained her bearded dragon to follow simple commandsâthey can't actually comprehend language or communicate with it, so I don't know that baby sign would have really helped.
Unlike human babies, who around two understand their native language fairly well internally even if they struggle with communicating their own needs and thoughts, apes fundamentally can't fully understand and utilize human language. A two year old deaf human using baby sign has a better understanding of their native language than an ape literally ever could of english or any other language. If you actually taught the apes and monkeys baby sign, and they could sign it with their very limited (afaik much more limited than even a two y/o human would be), and left them to their devices...they would only use the signs when they're interacting with humans to get something they want like food or a toy. There's never been any documented private speech, nor attempts for one gorilla to talk to another in sign, or the ape/monkey ever starting a conversation with a human without being prompted, and that shows they haven't learned anything other than the hand motions that make the humans give them what they want. That's it. A more coherent form of sign wouldn't magically make them able to hold a conversation, because the signs being bullshit isn't the reason the apes can't use language, it's just because they can't.
(Legit if you actually watch the head scientist at the Gorilla Foundation when she was talking to Koko, she legit had to ask Koko questions dozens of times before Koko responded in a way that made even a little sense and most of the time the scientist just gave up and made a joke about it. It's extremely obvious when you watch that Koko knew basically nothing and her handlers were making everything up.)
There's also a reason the Gorilla Foundation never hired any deaf workers. The Nim project did, but from what I've heard the Foundation was deeply opposed to hiring deaf people fluent in ASL, likely because they knew those people, like the ones in the Nim study, would instantly be able to tell that this is all bullshit and call them on it/go to the news. Abled folks with no background in sign, however, would have no way of knowing what they were being told was wrong.
So, Koko probably knew her name at the very least, and recognized words/signs for food, but she, like most non-human animals, could not actually understand or utilize language in the way a human can. We don't know why exactly, but overall there is something different about humans that lets us use language in a way no other animals can. And while some will debate this, language is far more complex than just word association.
I do reccomend the video, at the end the essayist says that it's silly to think animals need our language to communicate with us, because we've always been able to communicate without it. Your dog can't say the word outside or walk, but you and your dog both know what your dog wants when it sits by the door or sniffs at the leash hanging in the entryway. I can't explain to my cat that she needs to let me clean out her ear so she chills out about it, which sucks, but after a while she realized it feels better clean and now will "fake" scratch her ears in front of me and my fiancĂŠ to signal that she wants us to help her not be itchy. We've always found ways of communicating with animals that never involve them learning to speak a human language. I don't need my cat to be able to say she loves me because after being in the hospital for 2 weeks she wouldn't leave my side for days. And she knows I love her because I take care of her, buy her toys, play with her, cuddle with her, make sure her food and water bowls stay clean and full, all that. Neither of us needs to say it.
But yeah all that to say I really don't think the specific form of sign language was the problem, they could have tried to teach them any form of sign and it wouldn't have actually lead to them understanding language the way a human can. It's a grift that has been responsible for a lot of human and animal suffering for literally no reason.