Keychat is an app. It supports protocols and is open source but which other apps are compatible? To my understanding, WN explored signal and MLS, too and decided to go with more advanced tools to be able to rotate users in and out of group chats while group chats can scale to thousands of users, which would not be practical in Signal. WN so far is "only an app", too but the devs are working on dev kits for integration in other apps right now. It remains to be seen if Marmot will get picked up by other nostr clients but the feature claims look awesome.