An AcDb2dPolyline has as an elevation and a series of X,Y points of class AcDb2dVertex. The position() and setPosition() functions of AcDb2dVertex specify 3D locations in the ECS. The Z coordinate passed in to the setPosition() function is stored in the entity and is returned by the position() function, but is otherwise ignored. It does not affect the polyline's elevation.
The AcDb2dPolyline class provides the vertexPosition() function, which returns a World Coordinate System value for the vertex passed in. The only way to change the elevation of a polyline is using the AcDb2dPolyline::setElevation() function.