why is the article il required here and not in the sentence prior: Lui mangia riso
Okay from what I gather, Italians really like to use the definite article and you could use the definite article even if we wouldn't. The definite article can be used for a specific item or for a generality. So if you mean that "I eat (some) rice., I wouldn't put the definite article, but if you mean "I eat rice (the substance)." or "I eat rice (in general).", then you would put the definite article. This is unlike English which uses the indefinite form for generalities.
Actually you can make this sentence: Lui mangia il riso. You can make this one too: Io mangio riso
Rice, the substance, generally speaking, would also take the definite article in Italian, even though we would not use the definite article in English for generalities.
Try reporting it as also correct, but they would put the definite article for the generality even though we wouldn’t.