Is the OwnerId field in the Report object a polymorphic key or not?

So the OwnerId field in the Report object can be either a reference to a Folder, Account, or User, doesn't this make it a polymorphic key? And if so, why doesn't this work?

```
SELECT Name, Description, OwnerId.Name FROM Report WHERE OwnerId.Type = 'Report'

```

This is the doc I'm referring to for polymorphic keys: <http://www.salesforce.com/us/developer/docs/api/Content/sforce_api_calls_soql_relationships.htm#soql_polymorph_keys>