This. Metric us nothing to do with it, similarities or not.
1 Mil = 1 inch @ 1000 inches 1 Mil = 1 foot @ 1000 feet 1 Mil = 1 yard @ 1000 yards 1 Mil = 1 peckerjoule @ 1000 peckerjoules
Not exactly... A radian, or a radian/1000 for that mater, is a unit of angle not of distance. An angle can't equal a distance. In other words, units can't be mixed. The expressions shown are approximations not actual equalities. The approximations applicable because the angles are small.
Also, radian is the derived unit of plane angle in the SI units.