In zope.interface, you have something the following construct:

class InterfaceBase:
  pass

Interface = InterfaceBase()



Using the above Interface as a derivation base for your own classes, will make that instance a type derived class:

class IFoo(Interface):
  pass

type(IFoo)
-> Interface

type(Interface)
-> type


I wonder why this behavior is not documented in the official documentation, or at least, I was unable to find it there...

This doesn't work as you show.  What you probably meant was something like this:

class InterfaceBase(type):
    ...

Interface = InterfaceBase('Interface', (), {})

class IFoo(Interface):
    ...


which you can just as well do by using normal metaclass syntax.