> By the 12th century, this method of fermenting fish had travelled from the Mekong to China, and then on to Japan, where it was called narezushi.
It's a substantial claim that fermented fish and rice travelled through China to Japan. Food travels very slowly. If it travelled by land, it'll morphed with local ingredients. South China has more rice based dish. North China has less rice and more wheat in their cuisines. If fermented fish and rice travelled that far through land. It'll leave lots of traces in Chinese cuisines.
It's likely that Japanese got fermented fish and rice directly from South East Asia, without going through China. Japan has been trading with SE Asia region for centuries. Hội An is a city port in Vietnam with Japanese remnants.
Fish sauce travelled from Rome to Vietnam, through China.
Sushi travelled from SE Asia to Japan, through China.