Hi. No the nId is the Body Id (geobody id).
To go from this geobody id (if needed) to a treebody id we can use (this is confusing but for another discussion, for geobody and treeebody):
body = ExtAPI.DataModel.GeoData.GeoEntityById(nId[0])
treebody=ExtAPI.DataModel.Project.Model.Geometry.GetBody(body)
Erik