The main issue wasn't grammatical correctness, it was being grammatical at all. It's not surprising that an animal can learn individual pieces of vocabulary: anybody whose dog loses its mind when the word "walk" is mentioned, or watched meerkats for significant periods of time can observe vocabulary in animals.

Koko was intended to be taught grammar, specifically the ability to express new thoughts by combining her vocabulary in an ordered way. Despite Francine Patterson's best efforts to convince the world otherwise, Koko never achieved this.

There's some research that some birds understand grammar [1].

[1] https://www.youtube.com/watch?v=jmys2abx4co