I agree with most of Kenzi's argument, which I had not heard before.

One concern that comes to mind is that a Singleton is, by definition, an entity that can stop evolution at all lower levels. An AI that makes nanotech of the gray-goo variety and eats all living entities on earth would destroy all evolutionary levels. It doesn't need to expand into the observable universe to be bequeathed with singletonhood.

More generally: I can see many ways that an AI might destroy a civilization without having to after finishing it off depart on a quest throughout the universe. None of those is eliminated by the great filter.