Yes, I cannot but agree with that, since both words can be used interchangeably usually. However, I guess there is a difference, it's ambiguous though and Duolingo always wants us to be precise when it comes to interpreting certain words.
According to a dictionary,
밤 : 해가 져서 어두워진 때부터 다음 날 해가 떠서 밝아지기 전까지의 동안.
and for evening, there is another word you can use, 저녁.
저녁 : 해가 질 무렵부터 밤이 되기까지의 사이.
In your answer, evening should be accepted but the slightly different usage should be considered.
So anyway here's my thought.
When it is getting dark around - 저녁
It's already dark outside and pitch black - 밤
Hope it helps.
Vielen Dank für deine nette Antwort und Erklärung.
I agree with your comment, there is a difference between 밤 and 저녁, as you pointed out. It is my understanding that the usage of 밤 and 저녁 is a bit different from night and evening in English.
Your explanation helped me a lot. Thank you again and good learning.