That's a good point. Threema using standard libsodium cryptoboxes makes this easier to reimplement than these Axolotl-like protocols. Still, Wire has a bot API so you don't need to reinvent the wheel to integrate in a chat. Not sure that's any harder than using libsodium.

Afaik Signal doesn't have an API or SDK, there only seem to be third party implementations for bots.