I do... ;) This is 'a' as in 'a type of'. As you would in 'milk is a dairy product' 'an apple is a fruit' (you're right, I wouldn't say 'apple is a fruit' - maybe it's about the role of the noun?) 'gorgonzola is a cheese' 'water is a liquid' etc etc - am I wrong in all those too?
I know that in Russian the equivalent (basically identical) word would have this meaning. So is it me or is it Duolingo? :)