You're absolutely right to pick up on that, I did study the landscape and Matrix is closest.. biggest difference is fmsg is just messages - group like chats evolve naturally in the threads - but to get a message someone has to send you one. Group messaging platforms like Matrix, Rocket.Chat etc have concept of rooms/forums/channels i.e. groups, then have HTTP APIs to manage membership and sync messages.. fmsg just messages someone has to send you

Also fmsg being its own protocol can do novel things like to auto challenge during sending back to sender - can't do that with HTTP