You even give more credence to the author's argument than he does. He only suggests a possibility. The word is credulous.
Credulous? For not resorting to the "My Invisible Friend" explanation just because a particular non-magical scenario isn't nailed down? Shouldn't claims involving supernatural beings be the ones that have to be nailed down solidly?
No, actually, the word is "irreducible complexity," and the relevant claim is that IC systems cannot have evolved in any stepwise, naturalistic fashion. Therefore even a mere "possibility" (which you acknowledge) of a stepwise path, even by means of a "just so" story, is a refutation of the claim that this system must be attributed to "Intelligent Design" because it is "irreducibly complex".