Users searching for bimx to dwg conversion are typically trying to export or recover 3d assets from ArchiCAD's .bimx files (in this particular case to DWG), but that is not possible.
BIMx format is used for rendered 3D scenes, if you need the 3D model, you have to export the source file. ArchiCAD otherwise supports export to DWG, it is just not possible with BIMx files.