Marie Antoinette did not say “let them eat cake” when she heard that the French peasantry were starving due to a shortage of bread. The phrase was first published in Rousseau’s Confessions, written when Marie Antoinette was only nine years old and not attributed to her, just to “a great princess”. It was first attributed to her in 1843.
How does it predate her reign? Most wrong quotes come after the alleged author’s death
I already provided the link for you to check this, but it’s on the history list, under “Early Modern”.
The citation is this HowStuffWorks page.
This does not answer your question at all, but your confusion reminded me of this, which you might find amusing
https://youtu.be/FvfoS9tGYCU&t=550