I would think that IA is a function of the biz needs and (anticipated) site visitor expectations. Understanding these gives necessary context to the information that's being architected. In other words, I think there might be a prequel to this article.

Also, I'm not so sure the page model (i.e., site map) model applies in a CMS driven website world. I suppose it can't hurt (as part of the vetting process) but none the less my emphasis to clients is think in terms of content (e.g., copy, images, etc.) and not pages.