A PolygonMesh object is not geometry like Faces and Edges are. It is simply a convenient data container for points and "polygons."
The points are Sketchup Point3D objects.
The "polygons" are Arrays of Integers representing the indices of the Point3d objects. The polygon arrays have 3 elements (are triangles.) A negative index means the Edge between the points is hidden (soft/smooth.)
To construct the Mesh as geometry, you need to use Entities.add_faces_from_mesh or Entities.fill_from_mesh.