Not quite sure what marriage date you're looking for:
http://canadp-archivesenligne.paris.fr are the archives for marriages in Paris - within the 20 arrondissements (following the instructions given earlier)
http://archives.ville-saint-denis.fr/ are archives only for Saint Denis : click "etat civil" and then choose tables décennales, mariages, and a date range to check the indexes, or registre d'état civil instead of tables décennales to see the originals.
The bit you quoted from the Saint Denis archives just says that they've indexed records before 1792 (e.g. normally the tables are only for the civil records and parish records aren't indexed), and these records cover all parishes/establishments (e.g. hospitals).
If the family ever lived in other locations like Saint Denis that are close to Paris but not technically part of it as far as the archives are concerned, you'd have to check whether there were municipal/departmental archives which have records online for that area.
