"Men speak less than women."
Translation:Đàn ông nói ít hơn phụ nữ.
His confusion was the construction ít + verb + hơn + object. I would have expected verb + ít + hơn + object, which is indeed also correct but moreover the way things were taught.
Another case of DL doing a poor job of either not teaching properly and/or throwing at the students constructions not taught.