@oristarA,
Fin is the French word for end.
Fin de siècle means the end of the century and is usually used in the same way that "turn of the century" is used in English.
Fin du monde means end of the world, and is not a stock phrase the way
fin de siècle is.
The author was saying that the spirit of whatever times he or she refrers to (and you haven't given enough context for us to know--i'm not going to waste my time searching for it) were not the end of an era, but rather "end of the world," the complete destruction of the old order as was the case with the French and Russian revolutions.