If I understand the docs correctly, geodesicDistance expects longitude/latitude coordinates, so you may need to project the point coordinates to get a correct azimuth between the points. However (someone correct me if I'm mistaken), I don't think the azimuth you get from the geodesicDistance method will produce the expected result when applied to the web map that is displayed using the basemap's web mercator projection.
Maybe it is sufficient to just calculate the 2D angle between the two web mercator point coordinates? For example, this stackoverflow post covers how to do that. I'm not aware of a method in the ArcGIS SDK provides a convenient way to calculate that.