Emilian anaandika kitabu also needs to be accepted. It is irrelevant which lesson something comes from when you're practicing.
I agree, I even think it is a better translation. I would expect the usual tense combined with a plural subject, line Emilian huandika vitabu, which would qualify Emilian as a wrtiter. Writing one book is not a habit ...
Still they won't correct their problems. Fire the Kiswahili consultant!
There's nobody to fire, not even a volunteer anymore! You could address you complaint directly to DuoLingo, although this will turn out to be a useless exercise :(