I often find that the lower the word is in the hover hint, the less likely it is to be accepted. They are usually fringe cases (or wrong). The hint system really is a hint, not an actual translation. Looking at your discussions, I see you've already noticed this.
Only shows casaco as a coat.
Actually has a couple of references to sweaters, but the rest are for coats.
I think you are better off sticking to the common usage, especially if you like to keep your hearts:)
If you think you are right though, report it using the feedback. I don't believe that the staff at Duo use these comments to fix the sentences, only the feedback.
Looking at google images,
there are some sportswear, fleeces and hoodies. I suspect that is where casaco gets its "non-coat" meaning
Great answer! To add to it in a simplified way:
Fleeces and hoodies, as well as cardigans and sweaters, tend to be called "um agasalho" (a general purpose word for clothes that keep your torso warm).
"Um casaco" is indeed a coat or jacket.
A suit jacket is called "um blazer".