So the work-around works 😄
Theoretically the "Add location" feature should work no matter if the model is already geo-referenced and no matter how it is geo-referenced.
SketchUp adds some special attributes when the terrain is imported with "Add location":
ModelTranslationX -18855495.7977869 ModelTranslationY -215494081.152937 ModelTranslationZ -4278.33784800286 ZValueCentered -4278.33784800286
I have no clue what that means, but it's certainly missing if you only have the normal coordinates. I'll test it out or report the problem to Google.