I thought and assumed that the add-on also makes on fly the tessellation.

Some more details on what Burr mentions here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5875.3

Often times 3DM import plugins like this kind of take a shortcut and don't do any tessellation of their own. Instead they rely on a cached tessellation being present in the file. MoI doesn't write that because it significantly bloats up file size.

- Michael