As usual, you work on something for days and after you ask a question you answer it yourself.
Well, I would love a real explanation as to why this works:
The original class type that I was trying to use, to use the functionality of the IAssetManagerEditorModule, was deriving from UAssetManager. I made the class with the Unreal C++ class maker and it seemed to make sense to me.
Well, I tried to implement the same functionality with a class derived from UEditorAssetLibrary, and it is able to call the Get function without name mangling.
I have no idea why one class works and another does not.