The display is wrong, not the effect. They should have their damage listed as 5 or more appropriately 1x5 (avg 5). This is the way unit combat has worked for a while.. You could have 1 billion defense and still take 5 damage.
Looking at it another way, this unit type would have no threat rating, as they would nearly always inflict zero to even lightly armored units. Whether or not this is a balanced mechanic is up for debate, but the design was purposeful.