"The prince drinks red wine."
Translation:Le prince boit du vin rouge.
It is du because it is a general statement regarding a mass noun. It would need to read "The prince drinks the red wine". to mean Le prince boit le vin rouge. Either that, or the verb drinks would need to be replaced with a verb of appreciation such as likes - "The prince likes red wine" translates to Le prince aime le vin rouge.
Have another look at the T & Ns here: https://www.duolingo.com/skill/fr/Food
The need for a partitive article is not justified by a "general statement regarding a mass noun", which would be "Red wine is better in a stemmed glass" - applying to the whole category of "red wine in general", and translating to "Le vin rouge est meilleur dans un verre à pied", with a definite article.
"Le prince boit le vin rouge" is specific = the prince drinks the red wine (not the white one).
Partitive articles are used when the action expressed by the verb is performed on an unknown quantity/part/amount of a mass thing.