Yeah, no representation of that site as anything more than a site that happened to have a sitemap file that I could copy and paste was intended!
I'm not convinced that a diagramming tool ought to have built-in code to spider websites as core functionality, though Omni has opened the door to such requests by making OmniGraffle diagram disk structures and so on. But if you think it should, send in your feature request with Help->Send Feedback or email
omnigraffle@omnigroup.com and they'll take it into consideration when deciding what to do next.