From the documentaries I saw on the subject it was clearly the other way round. The researchers realised that the apes used sign language among each other, but at first this was not recognised as such because it was being used in a very fast and fluid manner and easy to miss.
As soon as the researchers realised the apes were using sign language (and had been all along), they set out to learn the apes’ signs for themselves in order to communicate with them in their own language.