The AppleScript dictionary for OF lists a property of project called effectively completed:
effectively completed (boolean, r/o) : True if the project is completed, or any of it’s containing projects or project are completed.
What are a project’s “containing projects”? I know a folder can contain projects, and a project can contain tasks, and a task can contain other tasks. How can a project contain projects?
This looks like a documentation bug. Projects and tasks both have the “effectively completed” property, and for a task, I suspect that snippet says “…or any of its containing tasks or project are completed.”
That’s sensible for a task, but you’re right — a project doesn’t have “containing projects.” Rather, I think a project is only effectively completed if that project itself is completed. Thanks for pointing this out!