This is the Olm protocol by Matrix, which uses the Perrin-Marlinspike Double Ratchet described at [1], and makes some similar cryptographic decisions to the Signal Protocol, but has a different wire format. As complete protocols, Olm and the Signal Protocol started off close but have since diverged in a number and magnitude of ways that I lack the expertise to comment on. Nonetheless, here's the same thing directly out of both Arathorn and Moxie's mouths, devs of Matrix and Signal respectively [2].
Despite valiant efforts on many people's part to lessen the conflation and confusion of protocols, this happens a lot still, but we can all try to do our part to be as accurate as we can.
[1] https://github.com/trevp/double_ratchet/wiki [2] https://news.ycombinator.com/item?id=11725652