I think the title is misleading. If I'm not mistaken it's not actual infinite scrolling, the only change is the search bar and search tools remain in a fixed position. It's a nice change.
The key phrase in the article is "setting the stage". Google hasn't switched to infinite scroll yet, but with a fixed header it seems most likely for them to do so soon.
My guess is that the pagination controls are there to accomodate people with JavaScript disabled.