The question has been answered in part in this post: How serious is saying "Je t'aime"?, with Papa Poule answering:
... you’ll know soon enough what it meant to her or at least how she chose to take it. If she responds with “Moi non plus” you’ll know that she's a playful “keeper” who’s ready to take it anyway you want her to. – Papa Poule Feb 6, 2016 at 17:41
Getting to the nub of my question: did the Serge Gainsbourg song Je t'aime…moi non plus change the meaning of the phrase je t'aime? If I recall correctly, before the Jane Birkin version was released in 1969, it was not a strong statement, and meant more like Je t'aime bien does today. Back in those distant days it took some flowers to get the message across.