What is the unique selling point that it has over Matrix?
replyMatrix addresses have a similar format, anyone can run a host, open protocol, domain ownership, interop... Threaded messages are supported AFAIK, the details of the crypto will be different but overall it feels like it is close enough that a new protocol will have a hard time having enough advantages to overcome the huge network effect (Matrix being one of the few open messengers that actually have some following already).
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
replyAlso fmsg being its own protocol can do novel things like to auto challenge during sending back to sender - can't do that with HTTP