Duo generally accepts numerals when you're writing in English, but not in Spanish. It makes sense if you think about it, because they are trying to teach us how to say/write Spanish numbers and just typing '1720' doesn't really teach us anything or demonstrate that we've learned how to say 'mil setecientos veinte'. But when we're typing in English, they assume we know English numbers already so just using numerals is fine.
If you hear and understand it in audio form then there's a higher chance you already know how to spell it out in full, especially since Spanish orthography is pretty easy and consistent. Your argument makes sense for the other exercise types, but in a pure audio transcription exercise you would typically expect that the numeric form would work, since that's the standard way of writing it. I'd say it's more important to build an association between the pronunciation and the number form than with the word form. You could still get your spelling practice in the other types of exercises.
But anyway, that's why I said "arguably".
In English we rarely say thousand when we talk about years. For example, World War Two started in nineteen thirty-nine, not in one thousand nine hundred and thirty-nine. Because there is currently no one hundred in the years it is probably more common to hear two thousand nineteen than twenty nineteen.