Yup, just to reinforce from the Matrix side: Olm is an independent E2E protocol which happens to implement the double ratchet (formerly known as the axolotl ratchet), but it's not Signal Protocol and it's increasingly divergent from Signal, and it's completely independent of Open Whisper Systems. For instance, we're defining different behaviour for group chat ratchets, called Megolm, following our own design at http://matrix.org/speculator/spec/drafts%2Fe2e/client_server....

That said, we've tried to architect it such that we could implement compatibility with Signal if Moxie and OWS were ever open to it. But it looks like we'll need to first prove that Matrix can support a rapidly evolving yet federated ecosystem without compromising UX or privacy :)