I'm going to guess that it's a verbal flow thing, required because scrittore starts with s+consonant (a bit like how in English we arbitrarily change "a" to "an" when it precedes a vowel). Testing this out in Google Translate with "lo" words other than scrittore (such as schermo, zio, studente, squalo), "uno" seems to be used for all of these.
hmm. Should duo also remove the sentence
la scimmia legge un giornale
While I agree 'it writes ...' would rarely be used in spoken English, it is a sentence that could be expressed by someone for some reason, perhaps even irony. I favor duo accepting any reasonable translation and including as many possible translations as acceptible as they can, so people don't lose their hearts for thinking about and exploring the possibilities in translation. Exploring the boundaries of the learning space is itself a learning technique for some people.