Truth Social is just Mastodon on the backend and an alternative Twitter-like frontend called Soapbox. Soapbox is indeed Elixir-based and open source.

Soapbox has a very weird history of first being forked from Gab for a feminist platform called Spinster(.xyz), then it got acqui-hired by Truth Social, then many of them left Truth Social to be independent. Soapbox(.pub) today is mostly abandonware, the team switched focus to building products on top of Nostr.

The amount of times it switched sides in its 5 or so years of existence has been truly fascinating and difficult to keep track of.