'Jadi' is the main verb here, not makan. It's not a word that can be directly translated into English (I think). It is a verb to show an action (in this case 'makan') that is/will be done, carried out or completed. I personally would translate 'jadi' to 'managed to' even though it sounds clumsy - but that's the closest I can get.
The English translation here might've thrown you off because the word 'jadi' doesn't have an English equivalent, so Duolingo gave us a loose (and not a very good) translation.
Other e.g.: 1. Dia jadi pergi ke pasar. - He (is) going to the market (after all). OR; He managed to go to the market.
- Kamu jadi ke Jakarta?
- (So) you (can go/will be able to go/are going) to Jakarta? OR; You managed to go to Jakarta?
The English translation here is wrong. Obviously written by non native English speakers. The "jadi" here seems to me (i could be wrong) to be like "ended up" or "in the end" as in - akhirnya mereka tidak jadi pergi - they ended up not going. So to say "mereka jadi makan ayam" as a question should be "are they going to eat chicken" (by extension - "with us"?). Its stupid to translate this as "so they eat chicken" (thats like asking if they are vegetarian or not - which no Indonesian would think is the meaning of this sentence). Mistake
Is the meaning: "Do they just (only) eat chicken?" [i.e. do they eat chicken and never anything else] Or is it "So, do they eat chicken?". [i.e. do they eat chicken, without implying that they do or do not eat other things] or "So, are they eating chicken?" [i.e. are they eating chicken right now]