1. Perhaps Kurzweil's hiring is largely a brand acquisition. The inventor's specialties in "machine learning and language processing" are important technological concerns the company has. It's original search engine was an improved algorithm enabling people to more easily find what they were looking for. Every Google product, including Maps and the self-driving car, are stamped with this ease of human-machine interaction.
My favorite example of this is the Google Doodles. They can be incredibly complex, but the presentation is so clear that we learn how to use them simply by playing with them. They're the perfect Google brand icon: a little machine that acts like it knows us.
In this scenario, Kurzweil is supportive, but more of an honorary engineer, an inspiration to the researchers, someone who might toss out an idea or two to fire them up but is largely out of the picture when it gets down to marketable projects.
2. Kurzweil's name is one of those that is widely known in certain circles -- and those circles can be pretty disparate -- but is still not a household word by any stretch. It could well be that Google just doesn't see this hire as being of interest to investors or to the public.
I can't discount this possibility completely, but on its own, I find it hard to believe. New hires are usually touted to the media as the best possible move for any company. Even if we had never heard of Ray Kurzweil, Google should be singing the praises of its new Director of Engineering from the rooftops.
3. Kurzweil's association with the topic of "singularity" could turn into bad publicity for the company. It is regarded at worst as lunatic fringe by some, even within the tech community. Microsoft
4. There is the final possibility that something bigger is indeed going on, a project or two in the works that Kurzweil help bring to fruition. Let's say there's a good chance it could materialize in six months but an equally good chance it will take six years, or won't materialize at all.
If I were Google, that would reason enough to keep my mouth shut now.
My best guess here is some combination of all four. The company is betting that Kurzweil's idiosyncratic personality and less-than-perfect name recognition will permit it to say as little as possible, thus avoiding fueling speculation about projects that may not pan out while also skating gently past any media volatility stemming from his involvement in the singularity movement.
At the same time, regardless of his level of direct involvement, having him there will undoubtedly spur the engineers in Google's labs to do their best work.