I actually took a go at answering this questions in a Short Answer to Simple Questions a few months ago.

Short Answers to Simple Questions | March 20, 2019

My answer was not really long enough or detailed enough for a proper answer here, so I apologise if I break the rules.

Basically, this is a really difficult claim to investigate historically. The modern Rice Theory comes from a paper from 2014 written by Thomas Talhelm, who taught English for 2 years in southern China and then for 2 years in northern China. He then went back to the US to do a PHD in cultural psychology, and wanted to study the differences between the north and south in China. The study used modern psychological methods and ideas to try to explain historical differences in Chinese development. An article in Science Daily summarized the paper by saying:

rice farming is extremely labor-intensive, requiring about twice the number of hours from planting to harvest as does wheat. And because most rice is grown on irrigated land, requiring the sharing of water and the building of dikes and canals that constantly require maintenance, rice farmers must work together to develop and maintain an infrastructure upon which all depend. This, Talhelm argues, has led to the interdependent culture in the southern region.

Wheat, on the other hand, is grown on dry land, relying on rain for moisture. Farmers are able to depend more on themselves, leading to more of an independent mindset that permeates northern Chinese culture.

It's a similar argument that Malcolm Gladwell used in some chapters of his book, The Outliers, to explain the differences between historical pastoral and agricultural societies. The problem is that both of these theories use incredibly subjective readings of culture, history, psychology, and research to form their conclusions. Large scale historical research on these ideas are scant, and often fall back on subjective or politicized responses to history.