Can the following be a possible translation: "He wrote also later about that" ? I interpreted the german sentence as he had written before and also later about that. The given correct answer seems to mean that some people wrote before about that and he also wrote about that but later. At least this is what i understand. I'm a not native english speaker.
In my opinion, interpretational exercises are too hard to accurately moderate for a relatively simple app like this: this app (again: simply) wants you to answer in the simplest way absolutely possible. Although not very pedagogical at first glance, this is dtill very valuable: being able to express yourself clearly, concisely, and in a compact way, will make your sentences carry a lot of weight - without boring and/or distracting your conversational partner.
The English sentence you suggest is grammatically incorrect. You could say "Later, he also wrote about that." Without context, in English at least, both sentences are ambiguous as to whether someone else wrote about the thing earlier and then later he did or whether he wrote about something else earlier and then later he wrote about that.