No, people use "duvet" specifically to refer to the blanket-like things that go inside duvet covers. Sometimes those are filled with down, but many types of filling are used these days. Back when the word "duvet" originated, down was probably the only filling used, but now that we have alternatives, it makes sense that the word "duvet" is still used to refer to non-down alternatively-filled blanket-like things, because language evolves.