"The composer is walking outside, and the director is waiting inside."
Translation:A zeneszerző kint sétál, a rendező meg bent vár.
I'm not sure whether it's good pedagogy to present a solution like that but honestly, it sounds quite legit and reasonable without any "meg" or "pedig" or whatever conjunction and it expresses basically the same. I wouldn't call it incorrect or incomplete, just less didactic.
You weren't marked wrong because of that. You were marked wrong for using "az rendező" instead of "a rendező" and Duo showed you a random correct solution that contains "meg".
While we are at it - they aren't quite equivalent but both can work as a translation here. With "és", it sounds like two rather unrelated statements that happen to be true at the same time. If you replace "és" with "meg", that's going to be worse, still the same meaning but a bit like you used "plus" instead of "and". Perhaps worse. If you put "meg" after the topic word - as you probably saw in the correction, "meg" being after "a rendező", you will have a contrasting sentence - you measure something (in this case, certain people) by something related to them (in this banal case, what they are doing). As if you made a very simple two column table.