That doesn't imply they can run WaveNet yet - for inference this net is sort of worst-case serial. Their TPU ASIC is almost certainly highly parallel, like a GPU - actually has to be that way for energy efficiency (which is it's claimed benefit).

Wavenet actually looks like it could possibly have been designed to run on CPUs in production, at least after they can further optimize it some. Sampling is super slow right now because it requires an enormous number of tiny dependent TF ops and thus kernels that have huge overhead for tiny amounts of work. A custom implementation could probably circumvent that by evaluating all the layers sequentially in local cache on a fast CPU.

Or they just designed it without much concern for production plausibility yet.