Does anyone know if Defold can be used without the UI? Eg. just running it as a library or using scripting to do everything the game engine UI can do.

Purpose is just to try to get an AI to generate code with it.

In theory yes, data is just stored in json and there's a CLI utility [1] for building. It's pretty awkward though.

[1] https://defold.com/manuals/bob/