It's so difficult being British when dinner means lunch and tea means dinner, it makes translating a bit more difficult
Middag and aftensmad can be used interchangably (afaik), so the answer Han spiser middag uden kød should be accepted too...
yes, they are synonyms, but this skill based on the previous skills, and they don't contain the word 'middag'. duolingo skills don't include every word what a dictionary does.
When do you say aftensmad vs aftensmaden, morgenmad vs morgenmaden, etc?
In this same lesson, I wrote "Han spiser mad uden Kod." and it was marked correct. Now it's not. This is NOT the 1st time that Duo has done something like this to my answers.