Wiktionary actually lists both words as sorta-cognates (under ‘etymology’). However, Wiktionary has a habit of not listing any dates. But, Trésor de la langue française informatisé says that the 1564 source uses the word in the meaning of removing windows of a house — while also noting that the French-root ‘défenêtrer’ should be used for this meaning instead of the Latin ‘défenestrer’.

In the typical blunt German manner, ‘Fenstersturz’ is simply composed of ‘window’+‘fall’.