You can say that, but Google has stated it's because federation was not used that much and that the spam aspect of it was the dominating usage. Which makes total sense and XMPP does not solve it, and in fact it's hard to see a solution for this problem that doesn't rely on a centralized solution.
Feel free to propose one. Because if you can solve it for XMPP, you can probably solve it for SMTP and become a billionaire for fixing spam.
