MLow: Meta's low bitrate audio codec 2 years ago
webrtc engineer here...
You are right that OPUS already hits the ball for most audio applications. 20kbps is pretty good, and 30 kbps is almost perfect.
Meta has tons of Indian and South Asian users with insanely poor network. It's already using 60ms/120ms packets (16.6/8.3 packet rate) for those people, but still, some calls are still bad.
What this new codec does is to further make the call accessible by bringing down the bitrate, which seems to help their DAU.
Nitpick: measuring packet rate on your device doesn't usually work. I think meta (as well as most other voip providers) is using DTX, so when user doesn't talk, packet rate is 2/sec.