They both are - from their different points of view. From an administrative perspective Réunion is an external department of France, but a statistician may choose to present data differently if lumping everything together may confuse. As an example this wikipedia page separates out “Metropolitan France” from “Overseas Departments/Regions” so that the “answer to the question that most people were probably asking” is not complicated by “the technically correct answer to the question that most people were not asking”.
I still don’t see what this has to do with Micronations. The PDF (that I keep referring back to so often that it’s turning up in my browser suggestions now!) covers “how things can be culturally sensitive” and how de jure definitions may vary from de facto ones.
Other definitions that have been suggested (not entirely seriously, allegedly by Frank Zappa) don’t really work - a map of “countries based on having at least one beer” would have significant omissions and yet would include Sealand..