From what I can gather FeatureLayer does not support sub layers. Why not?
The workaround is implementing the layer as a MapImage layer, I can still add a popup on it. But now I lost search! The search widget only supports FeatureLayer and this Locator thingy.
Its a bigger workaround implementing this as a MapImageLayer, for making it hold a lot of data as if were a FeatureLayer to be able to set up the search source.