Having read the short article it's pretty clear that he was simply saying that if you have an indoor nest camera it would be proper etiquette to tell your guests that they are being recorded on your device. From how I read it, he wasn't saying to let your guests know that Nest/Google might be listening.

Personally I'd never have one of these devices in my house but I would appreciate it if my friends gave me a heads up if I was in their house with one.