Hello Joel,
We appreciate your use of ArcGIS for AutoCAD. You are correct in that ArcGIS Online does not allow a Map (service) Layer that would compliment a Feature (service) Layer like we see with ArcGIS Enterprise. So, when you want both a deployed ArcGIS Enterprise is rather handy to have around.
However, if you are after Image Layers from ArcGIS Online you can publish these while also using them in ArcGIS for AutoCAD when you publish them as a dynamic Service (step 3) as opposed to a tiled layer.