This is a very beautiful study case for open source projects - someone needed a client for the niche OS (sorry Ubuntu Touch), so they just wrote it. It seems like they've done so without much prior experience.

As for Delta Chat - I've tried using it previously and I have a vague memory it messed up my inbox a little. Is there a simple way to separate the chats from my normal emails? I don't want to have a new email in Thunderbird for every instant message I get. Is there a solution to that?