White Noise is also just an app, from what I've gathered. Plus, when it comes to group chats, it seems to me that on the protocol level, Marmot it is trying to do the same thing that Keychat has already successfully done - facilitate large group chats using MLS with nostr relays as the transport layer. I might be totally wrong though - if so, I would be very grateful if you (or anyone else reading this) could point me to some resources explaining the difference.

Keychat’s one-to-one messages and small-group messages are encrypted using the Signal protocol, while large-group messages are encrypted using the MLS protocol. Marmot (WhiteNoise), on the other hand, encrypts all messages—both one-to-one and group messages—using MLS. It’s worth emphasizing that for one-to-one messaging, the Signal protocol is more efficient (and therefore more secure), because the new public key needed to advance the DH ratchet (which provides post-compromise security) is carried in the header of normal messages, without requiring extra messages to transmit a new public key. In Keychat, as long as the two parties exchange messages back and forth, the DH ratchet advances automatically. By contrast, if MLS is used to encrypt one-to-one chats, advancing the ratchet responsible for post-compromise security is less efficient and requires separate messages to transmit new keys. This is largely because MLS was designed with large-group messaging as its primary use case. MLS is built for large groups; one-to-one support is a byproduct rather than an optimization target.