They say Google home is an IoT device but it has the ability to control other devices like lights, Air conditioners, etc. (in smart homes) which is more like a base station in the Wireless sensor network. Or is Google home an integration of both the technologies together? I think IoT devices are connected to the internet directly. But in the case of smart homes, why are they using Google Home or Echo, etc. ?
This largely depends on the definitions you choose. The criteria for what makes a device an "IoT device" is contentious at best. Most definitions would call a Google Home an IoT device if you carefully read through the definitions given, but not everyone agrees on the same definition, so it becomes rather nebulous.
We can also look at how a wireless sensor network is defined, particularly:
The base stations are one or more components of the WSN with much more computational, energy and communication resources. They act as a gateway between sensor nodes and the end user as they typically forward data from the WSN on to a server.
I would tend to say that a Google Home doesn't fit this definition well - a crucial aspect is that other sensors connect directly to the base station, like a hub. While a hub like SmartThings certainly does this, generally a Google Home doesn't directly link to devices and rather sends messages via remote servers.
Ultimately you will need to consult the definitions you want to use to check - but remember that definitions aren't always agreed upon!