The quoted claim is false. I use Google Voice on Android. There's an option to "make all calls with GV" in which case all your outgoing calls go to the corresponding proxy number. There's also an "ask every time" option.
Although I'm pretty sure doing that in android still uses your mobile number and cell network to make the connection, not the WiFi or data connection. So it's still not true VoIP/WiFi-calling like it is on the iOS version of hangouts.
