After further investigation, I was mistakenly confusing MinimalAPI with EngineAPI. If a class is marked as EngineAPI, having the include statement for Engine.h allows it to be used. For something that is marked as MinimalAPI, this means that it is not available outside of source code and can’t be extended. If you are using a source version of the engine you can change MinimalAPI to EngineAPI in the PostProcessVolume.h file.