Though XMPP had to deal with a number of paradigm shifts: images, markup, audio and video calls, persistant history, disconnected operation, security, threading, multi device operation. And I probably forgot a few.
Today messaging is much more established. So the matrix team has many more examples of features that are in common use and how other people designed user interfaces.
So it may be harder for Google to win through engineering. They may just break other matrix clients on android.