Good question. From what I've seen on other discussion posts, I can say that putting the "the" before the word isn't always done; for many people, it's a habit, and it is understood by others. But not everyone does this. DL uses both forms, with the "the" and without, but when you're actually in Italy it's optional. People will understand you anyway.
"Meat or fish" makes perfect sense because yes, in the culinary world, they are different things. If you look at a menu, and the menu was divided into sections, it would have a section for meat and a different section for fish. This may not be how everyone sees it but it's still good to understand that's how it works sometimes.
I am of Italian descent. My parents and Aunts and Uncles never really taught me Italian. They used it to keep secrets from the kids in family gatherings. The words and phrases they used and that I remember are spelled nothing like i thought they should be spelled ( in high School I had Spanish. Italian was not offered) and what I hear is nothing like I used to hear. My family shortened every word. They would tell me that people from other regions don't know how to speak "real Italian". I'd hear arguments over not understanding what people from a few blocks away were saying.
I grew up hearing, Sicilians don't speak Italian, Venetians don't speak Italian. I can't believe how difficult this language actually is. How does the country function? But then I realize that when I go to the deep South I have difficulty with English or I have had problems when I visited England. Eventually we figured out what was being said. I think that we should not take Duolingo so seriously. If you travel to Italy and you get the locals to speak slow enough, you'll be able to communicate.