There is currently hardware available that will allow you to connect your phone sytem directly to the GV network. GV Jack is a software solution which works in this fashion with MJ hardware, for instance.
»"GVJack App" Converts MJ Dongles To Google Voice
So, whether or not the reviewer is only using GV through other phone companies I'd say it counts. And fwiw, been using GVJack with GV for about a month now and have been very happy with the results.
And to the OP, thanks for taking the time to review.