It is essentially the same, lunch is usually a bit more formal. For example when saying you're going out to lunch with someone, I would use 'lunch'. Most of the time, you could use them both. As far as Duolingo goes, I haven't encountered it that much so I can't say anything for this program
Generally you would omit the aricle as you would with any noun in a general sense:
Grass is easy to maintain.
In general, grass is easy to maintain.
The grass is easy to maintain.
This specific type of grass is easy to maintain.
So normally you wouldnt say he is eating the lunch, but you might in certain contexts.
Let's say I brought John lunch from Taco Bell. He wasn't there when I dropped it off so I call a little while later to ask if he ate it.
Well he is eating the lunch now.
He's eating the specific lunch I brought him.
Do you find that studying a third language through a second one (as Dutch through English) helps you with the second language as well? I started the Catalan (from Spanish) course after completing the Spanish (from English) tree, but have since gone after other languages instead...
'Eten' is a verb, and when verbs are used as nouns (i.e. the walking, the writing) they receive 'het' in Dutch. The fact that 'middag' is put in front of it doesn't matter, the article follows the gender of the last word in a contraction. Male and female words get 'de', neuter words get 'het'. 'Lunch' is a male word in Dutch, thus 'de lunch'. 'Middageten' is neuter, thus 'het middageten'