Let them eat Ice cream

Gabriel was at Jack’s agency.
“Hi Gabriel, how can I help you?”
“I’ve got a chance to buy a diary that’s 225 years old.”
The French Revolution was a subject of interest for Gabriel Dupont, a French boy. His family had a French sword (authenticated by Jack) it dated back to the revolution.
“Let me guess”, said Jack “Ernie?”
“Oui.”
10 seconds and Jack was poring over Ernie’s “Diary” that was from the French Revolution.
“… qu’ils mangent du gâteau. C’est indifférent, la façon dont elle a dit ça.” Jack read out. “What does that mean?” “The writer is referring to Marie Antoinette’s quote “Let them eat cake.” It says the writer thinks she’s indifferent.”
Ernie announced: “It’s yours for only sixty pounds!”
“There’s no way you’re buying that”, said Jack “Sixty pounds? C’est faux!”
Why?