PGP makes no claims of being an "end-to-end encrypted service", because it's not a service. It's an end-user product. It doesn't have to "solve this" problem, because that's not what it's for.
The question in this thread is whether iMessage can offer secure interop. The answer is yes. They just need to use an open protocol and that protocol can use tools like PGP to encrypt messages end-to-end.
Your claim that both the sending and receiving application need to be "controlled" by some entity for it to have "real" end-to-end security is non-sense.
The question, from tomrod, was "How does PGP solve this?"
Nothing to do with iMessage. I was answering a specific question on a tangent thread. If you want to argue with me about iMessage, go to one of the posts I've made about that on this article. This thread is about PGP.