A cardigan is a type of knitted garment that has an open front. Commonly cardigans have buttons: a garment that is tied is instead considered a robe. A more modern version of the garment has no buttons and hangs open by design. By contrast, a pullover does not open in front but must be "pulled over" the head to be worn. It may be machine- or hand-knitted.
Robes have buttons.
Based on the context, the statement that cardigans commonly have buttons while a tied garment is instead considered a robe implies that robes do not have buttons. Therefore, "Robes have buttons." is an incorrect statement. Maybe the system was confused due to the proximity of the word "buttons" to the word "robe".