correct; Matrix doesn't implement Signal Protocol. We use the Olm ratchet as a 1:1 ratchet which is an independent implementation of the Double Ratchet (aka the Ratchet formally known as Axolotl), and have added our own group ratchet called Megolm to handle the semantics required for Matrix.

In terms of how it compares to Signal: we are still in alpha (although Olm now ships on the develop branch of matrix-js-sdk and dependencies), and so obviously Signal is infinitely more mature. However, we are getting the core ratchet publicly audited during July before we push all the E2E formally live, which should give a better idea of how reputable to consider it.