Visit a Catholic church in France and realize it exists mostly as a museum, catering to a few elderly widows in black. Churches are maintained, like castles and palaces, as relics of a glorious past.
Being Catholic in France is like being a European, something you take for granted without putting much energy into it. The church is where you are married and buried and, if you're really faithful, where you go on Christmas and Easter. It's more a part of your ethnicity than a faith. This is largely true across Europe.
Mosques are built because there aren't so many. Churches aren't built because there are a surplus; some are being closed or sold off to become mosques.
The point of the article is that your preconceived notions about religion in France are wrong, which could be true if you've not visited there or read about it.
