Sorry for a late reply. The Dialogue Designer is an editor that can be used to more easily create dialogue files needed for this addon. The file format it uses (JSON) is quite complex and writing it by hand would be difficult, so I made the editor to make the process easier (but the editor is optional).
Moreover, this system is for Godot 3 only, and the Dialogue Designer can be used with any game engine. I hope that clears things up.