So the thing is that while Double Ratchet is a good encryption algorithm, MLS is a flawed implementation of it?
An essay explaining why we don't plan to use MLS: https://www.poberezkin.com/posts/2025-08-12-mls-the-naked-king-of-end-to-end-encryption.html
A sidenote, is that Nostr's Whitenoise avoids its main problem.
TL;DR: MLS security model is "Trust me bro".
Discussion
MLS uses ratcheting trees, it's different from Double Ratchet.