My understanding of the situation has been that most of the FaceTime technology is indeed in the open just because they mostly used already existing technology.

My understanding of the situation has been that most of the FaceTime technology is indeed in the open just because they mostly used already existing technology. What is not in the open is the cryptographic key that is used to sign all the FaceTime client keys. So technically you could create a FaceTime compatible client by following open specifications and filling the gaps by reverse engineering, but Apple devices wouldn't want to talk to it without Steve's cryptographic signature.