I won't argue against it, but "coined" might lead a bit too far

The one source specifically calls it a neologism.

Also, I just ran a search for libertaire in my French pdfs of Justine and La Philosophie dans le boudoir and came up with nothing.

Lots of libertin and libertinage, no libertaire.

Admittedly, this is not Sade's entire corpus.

Not sure I feel like wading through the entire German Sade entry to find the relevant info. My German is a bit rusty these days. Can you help by providing the appropriate excerpt?