fsweetser,
Honestly, you are at the mercy of that manufacturer's promises of not causing interference. They never mention Zigbee, but.... http://en.wikipedia.org/wiki/IEEE_802.15.4
If the device operates in the same spectrum as wifi, at minimum there is the potential for interference, and decreased performance because it does not "understand" or operate by the same rules as wifi. If they choose to change their protocol for some reason and it negatively affects your wifi, you would be stuck with poor wifi.
Maybe someone can chime in on their experience with these specific devices to provide more perspective.