Ah ok. I think the confusion on your part can be clarified by thinking of not loading HDAs directly (as you would in Unity in the non-PDG workflow), but rather think of it as working with cook outputs of your TOP nodes and loading the generated .bgeo files. When an HDAProcessor is used in TOPs, it's only active when it's being cooked, but you can't really access that HDA from Unity because its running in another process somewhere. Rather, you'd wait for its output file(s) and get the attributes by loading that file, which is usually a .bgeo file (for Kenny's tutorial we used bgeo files).
PDGAssetLink does this automatically using HEU_GeoSync.cs. Take a look at HEU_PDGAssetLink.cs::LoadResults where the HEU_GeoSync.cs is attached to a gameobject, and StartSync is then called. You can then follow the rabbit hole, and you'll eventually end up in
HEU_ThreadedTaskLoadGeo.cs::DoWork
which does the loading of all geometry. You can attach your attribute querying code in there somewhere to get the data from your generated bgeo files.