Is it inconsistent? In another lesson, Duolingo says, "These trousers are ..." but accepts only "Celana ini ..." and rejects "Celana-celana ini ..." or "Celana-celana panjang ini ...".
When we want to lift a lot of heavy things and feel tired, we say, "Barang-barang ini berat". The word " barang-barang" itself implies a lot of "barang". It's similar to "buku-buku".