Koko, as with most "signing" apes, was "taught" modified ASL (bc their hands are different and they physically cant make all the same signs we can) by hearing scientists who did not speak ASL. They would learn a few signs, and then teach them to the apes, who would associate signs to objects and rewards.
The most jarring thing was, the apes are completely unable to learn grammar, and would say things like "Give orange me give eat orange me eat orange give me eat orange give me you" (actual quote by Nim Chimpsky), which their handlers would interpret as a sentence, when in reality the apes are simply mimicking signs in hopes of getting a reward. Those hearing handlers would see things like "Nim eat" and "eat Nim" and intepret those as equally meaning "Nim wants to eat".
More damning, the lack of understanding of ASL by the hearing scientists meant that most of what Koko and Nim Chimpsky "spoke" was purely the scientists just seeing what they wanted to see. A Deaf person was brought in to interact with Nim, and they were instructed to not give him any food until he signed "food". They spent hours with an increasingly distressed chimp who did not sign anything, but Nim's hearing handlers would see him move his hand close to his mouth and go "oh! there it is! he signed it!", and while they spent the whole day signing, they didnt see Nim signing back.
With Koko, her handler would claim Koko would sometimes mix up signs like "need/knee", "I/eye", "people/nipple" because they "sound alike/rhyme" but... they don't. Those words rhyme in spoken English. They don't rhyme at all in ASL. Koko wouldn't know those words rhymed in english because she DIDN'T speak english, she "spoke" modified ASL. Of course, as the scientists did not speak ASL either, they didn't realize it, and just assumed random movements Koko meant were signs, and tried to think what she "could have meant instead" by thinking of what words sounded like the ones equivalent to what she had just "signed", even though an ASL speaker would not make a mistake like that.
I'm not even going to get into the fact that almost all of what those apes signed was due to direct prompting from scientists, the fact that they did not use language when alone, or the fact that most of what they answered was complete gibberish (which resulted in videos like Koko's climate address (yes, really) having to be heavily edited and cut to make it seem like she was actually speaking anything that made sense).
One really nasty side effect of this was like. The amount of hearing people who decided to try learning ASL and other sign languages because of the vague possibility of being able to communicate with apes, instead of, you know, the ACTUAL possibility of communicating with and appreciating Deaf people. (one person even said that Koko inspired them to learn ASL so they could communicate with their deaf friend, like... why the fuck did your FRIEND not inspire you to learn ASL??? did you really have to wait for a fucking gorilla failing to learn sign language to think "hmm, maybe talking to my friend would be nice!"??????)
The talking ape experiments helped cement in hearing people's consciousness the idea that ASL, and sign languages in general, are just poorly transcribed forms of spoken English that can be easily learned even by a chimp, instead of complex, independent languages with their own histories, cultures and internal variation.
