No, barley was a lower grade poor-people flour. So it's just saying that people will have to settle for less.
Since then it means they're replacing an argument about scarcity with an argument about economic subordination, ie get into debt to buy cake if you're out of bread so you can buy cake at 1000x the cost, do whatever we say to not die. Become Marie Antoinette Bourbon's slave.
A modern form of that is American medicine, that's cake. They tell Americans they're getting the best medical care in the world, so it's OK it's the most expensive. In fact it's the opposite, it's the most inferior because it's the most expensive, the less you pay the better the results, with Cuba achieving the absolute best results linked directly to the absolute lowest prices. Like religion, the less money you have the more devout you can be, prayer and fasting.
“Let them eat cake” is an expression that captures the ignorance and disconnection of elite from the working class and poor.