That means the "marriage" isn't recognized by the French government, nor by most other governments. I don't know where such marriages are recognized, since such an arrangement would be convenient for all of us wishing to marry up.
I might wish (with the approval of my parents) to be married to Pippa Middleton, for example. But duh! If no one tells her of this marriage, and if no one knows whether she consented or not, the marriage is a mirage.
A mirage! As was much of the news coverage of this horrific event.