You can get meshes from Breps in rhino3dm (also the .NET version) only when you save the 3dm file in Rhino with one view in a mode that creates render meshes - shaded or rendered mode should be ok. You can use the GetMeshes method on geometry to retrieve the render meshes.
The meshing code is not part of rhino3dm and will not be.
The display meshes need to be created in Rhino and then the 3dm file saved in Rhino. rhino3dm (outside of Rhino) will give you access to those meshes but won’t let you create them.
-wim