The big difference between MLS and Signal Protocol is that MLS is tree structured (it began with a tree key derivation algorithm called ART, and now uses one called TreeKEM). An MLS group is structured as a binary tree with communicants at the leaves; pairs of communicants share an addressable key, as do pairs of pairs (forming subgroups) all the way on to the root, which computes a key that addresses the whole group.

Signal Protocol is entirely pairwise. A Signal group is a set of communicants who have established N^2 pairwise ratcheting relationships with each other.

See my other comment about this being a way to achieve all the nice properties of the Signal protocol in an efficient and scalable manner for group messaging instead of just 1:1 messaging.