I think I've heard somewhere that David J. Peterson doesn't like using grammatical articles.
Would 'They are a woman' not work as well? 'Issa' doesn't seem gender specific and I haven't seen pronouns for 'She'.
That wouldn't make much sense in English either. It could be "they are women" but then the words in the sentence would change their endings to reflect plural form. I'm guessing it's something like "ābre issi", but not entirely sure (just got started, haha)..
'They' can be singular. It's just making it gender neutral since 'issa' can be used for many genders. 'They are a woman' isn't grammatically wrong and seems to fit the singular better.
If they wanted plural form, your example could work though.
I too use "singular they" all the time and I'm sure it's understood by most speakers, but it IS not an entirely standardized/adopted grammatical structure and hence possibly not recognized by Duolingo (maybe it can be tho, or if enough people report they might even change that).
Oh, and by the way, plural nominative of Ābra is Ābri (-e ending exists but it's singular accusative, as I've come to learn by now).