MLS would be the primary standard for group messaging these days with the usual guarantees right? (PFS, backwards secrecy, etc) As I understand it from the RFC, large groups was an explicit design requirement and costs are supposed to be asymptotically logarithmic with group size, so I don't see why it couldn't be used. I feel like Colibri (based on their page) just doesn't believe it's there problem, which seems... irresponsible.