Now you've done it. I had to go find out if my memory is still working. ;-)

From what I remembered reading a couple of years ago, someone in the 19th century was writing about the French revolution and the writer made up the quote to "personalize" the out-of-touch reality that the French Royal couple lived in. That is, no one said it at all.

I was not too far off in my memory. Here is a link to more info:

https://www.britannica.com/story/did-marie-antoinette-really-say-let-them-eat-cake