Tasks in Marvin can't be infinitely nested in that a task can have subtasks, but those subtasks can't have their own subtasks. Further, subtasks (currently) don't support labels or notes. Therefore a project, which can can infinitely nest subprojects (and it and its subprojects can have tasks with labels, notes, and subtasks) cannot map cleanly to a task. This means information would likely be lost while converting a project into a task.
Maybe in the future we will make it so that Marvin can determine whether information would be lost and only allow you to convert projects to tasks when it wouldn't. But this is rather complicated and potentially confusing for users.