I could say that there is not such a word like "one" replacing the noun in Greek. No, μάλλινο is an adjective, woolen here, but without a noun it is a noun. Specially in plural, τα μάλλινα, it is all woolen clothes or staff that one wears during winter and are made of wool. As a native speaker I have not a clear sense when we should use "one" in English, the reason it is not used in Greek.
"For the adjective meaning made of wool, woolen is the preferred spelling in American English, though woollen appears about a tenth of the time. In the main varieties of English from outside North America, woollen prevails by a large margin. Both spellings are common in Canadian writing." https://grammarist.com/spelling/woolen-woollen/
Canadian Oxford prefers "woollen."
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This sounds like something to notify Duolingo staff of directly: