Soup Emporium on YouTube has a fairly detailed and compelling (at least to me as a lay person) video about Koko and whether she could truly speak. He argues tha

Soup Emporium on YouTube has a fairly detailed and compelling (at least to me as a lay person) video about Koko and whether she could truly speak. He argues that, while she could perform signs correctly, there's little evidence that she understood them in the context of actual language, and rather saw them as tools associated with certain outcomes. For example, signing "food" when you're hungry is a form of communication, but so is a cat meowing loudly near its food bowl. It is a large leap to then claim that this communication approximates language simply because it uses the same tools.