Here's why I disagree: smart human beings evolved from not-very-smart primates.

That proves there is a natural process by which greater intelligence can be created.

Therefore there is no reason that an even greater intelligence cannot be created with help from man. And for singularity's sake super-human intelligence doesn't even have to be an AI. Genetically-engineered super-intelligent primates would do the trick as well. The idea is that once you've created something smarter than yourself, not matter how you did it, it will then be able to figure out how to make something even smarter. And so on. That's the singularity.