From their mastodon:

so my goal with sup. is to make a federated snapchat/whatsapp that is compatible with most platforms. This was originally going to be a Pixelfed specific Messenger-like app, but then it hit me - this is a perfect opportunity to create a sub project that could be eventually handed over to a trusted org.

Seems like a fine ambition but Pixelfed is still in beta and buggy as hell. I’d like to see that improve (so that more people will want to use it) first.

This guy has like 6 different projects going on (Pixelfed, fedidb, Kbin app, Messenger app, etc.). I just wish he would focus on like one or two things instead.

seems interesting, but honestly I don’t trust my mastodon instance they’ve already been raided before lol