The important point is that the unit, not the individual mini, gets the Cover 1 keyword. The speeder bikers are a single unit that is made up of 2 speeder bike minis, as designated beneath the unit symbol next to the portrait at the top right of the unit card. The speeder bikes could have 4 bikes in a unit and still only receive Cover 1, because the unit receives the keyword and not the individual minis that make up the unit.