It's separate: MLS and Double ratchet are different solutions to the same problem. MLS is more complicated, but better suited for large groups.

awesome, thank you for the clarification!

It could also be used in a 1-on-1 chat. Does it essentially become a "normal" double ratchet in that degenerate case? @JeffG

MLS works great for 1-1 chat. There’s slightly more cryptographic overhead than a signal double ratchet but really it’s almost negligible. And MLS makes turning a 1-1 chat into a group trivial, which is something signal protocol can’t do efficiently without Signals servers in the middle.