In a debug session, you need to set the local site manager before attempting this. Try:
>>> from zope.component.hooks import setSite
>>> setSite(app.Plone)
...prior to attempting to check if IReferenceable is provided by the object. The reason that this is necessary is that Dexterity uses something called an Object Specification Descriptor that looks up interfaces dynamically from the Factory Type Information of the type, which is site-specific (you cannot retrieve site-specific configuration without first having the local site configured for lookups).