That's exactly the same thing. Once you own most of the implementations running "the standard" you modify it and close it so that nobody can follow your path.
It does not matter if the said modifications are called "G-XMPP" or 'Hangouts'. The result and logic are the same.
And then again.. you didn't get the point.
SSH and Telnet do the same thing, but nobody killed off telnet by getting everyone to use their implementation of it, then kill it and propose SSH as alternative.
Thats what Hangout does. Name 5 of your friends without a google account who use internet & mobile phones. See what I mean?
They were using XMPP. Everyone could talk to them using XMPP.
Not anymore. Wanna talk to them? Gotta use hangout, no way around it.
It does not matter if its based on the same _code_ what matter is that it offers the same functionality AND kills off the previous product by using their market share.
It does not matter if the said modifications are called "G-XMPP" or 'Hangouts'. The result and logic are the same.
The rest is a useless fight on semantics.