I don't know where you found that story about oven waste being called "cake" but it is patently false. The supposed quote from Marie Antoinette, which is probably apocryphal, has her saying "If the peasants have no bread (pain, in French), then why don't they eat cake" (brioche, literally "sweet bread" in French) and it was intended to show how out of touch she was.
That is exactly what I thought for years. I can't remember what triggered me to research it but my OCD manifests in the strangest ways.
I don't know where you found that story about oven waste being called "cake" but it is patently false. The supposed quote from Marie Antoinette, which is probably apocryphal, has her saying "If the peasants have no bread (pain, in French), then why don't they eat cake" (brioche, literally "sweet bread" in French) and it was intended to show how out of touch she was.