Yeah - Hilary Mantel uses it in her novel as though it was contemporary. It sounds to me like it could be contemporary or soon after, though apocryphal - like a deadlier version of the many quotes attributed to Oscar Wilde that turn out to have no definite source. Buchner makes up a lot of Saint-Just's dialogue, but would the severed head/Saint Denis line immediately spring to the mind of a presumably protestant young German, in the way that it would to a catholic Parisian gossip-monger? That's what makes me think it's French, even if it was made up.