I’ve never thought there was much behind Combier’s claim. However, another distiller, called Saintoin, advertises a triple sec liqueur almost ten years before Cointreau’s supposed invention. It’s described as lighter and more delicate than traditional curaçao.
A 1895 biography of Edouard Cointreau, published in a biographical dictionary of France’s greatest industrial figures, had this to say about the ‘invention’ of triple sec:
It was a reaction against artificially coloured and heavily sweetened curaçao liqueur. This had to be light, “triply perfumed” and white.