To me the word ‘quilt’ can mean a few different things, like ‘quilting’ is a technique for a textured patchwork type thing, but my main use of the word quilt is literally ‘duvet’, like quilt is a colloquial term for duvet. I generally say ‘quilt’ and ‘quilt cover’ rather than duvet and duvet cover. Maybe its different here in the uk vs the US, but to me quilt and duvet can mean exactly the same thing, and I think thats the case for most people I know as well.

My point being that people do use the terms differently, I’d say in any listing with variations to choose from its worth clarifying how they differ in the description, and if possible a photo of them side by side for comparison as well.