My guess:
It's not old fashioned ethernet when carried on the fiber. Yes "ethernet" packets but the packets could be transported along the fiber ring by a token ring type protocol from node to node. No CSMA/CD along the ring. That's how we did it in 1985.
I wonder if might be similar to FDDI (and later CDDI) rings, which is deterministic by virtue of being a token-based network (they mention Etherloop uses TDMA), and allowing for loops in order to provide redundancy, but their implementation of Etherloop[1] uses ethernet-style packet frames, allowing for ease of existing devices to digest the packet payload?
[1] Interestingly "Etherloop" was DSL'ish last-mile data delivery technology...