I have passed your question along for review but it might help to share what you are trying to accomplish as well here so a helpful recommendation can be offered if the API is not functioning as desired.
Well I was hoping this would fire an event when Marks were highlighted by selecting a dimension. Other than that, the API documentation makes no mention of this event type so I wanted to know how to use it in case I do have use case for it.
Selecting is a different event than highlighting in Tableau (even though they look similar).
Highlighting happens when you have a highlighter dashboard object, or turn on highlighting on a legend, or create a highlight action.