In fact it’s the most factoid it could be, an outright falsehood shared as a fact.

I hadn't ever thought to look it up over the last 10-12 years! Imagine that.

Corn beats it out, and by some reckonings so does rice. Pumpkins too.

There are a few more exotic plants like tigernut and duckweed that are supposed to be really high, but not many people eat those as a staple crop. Palm oil and sugar cane are supposed to be super high too, though you probably don't want to be eating huge quantities of straight oil and sugar.

Finally there are a few tropical trees like jackfruit and breadfruit that produce enormous quantities of calories once mature. They have a huge advantage from their large leaf crowns and root systems (that they don't have to periodically regrow like annual plants) + the tropical weather allowing production for the entire year.

Most likely also because of their food. Diabetes, gluten intolerance, food allergies, vitamin deficiency from eating way too much of the one thing that grew and didn't die that harvest. Food related issues were all rampant, people just died of them.