> (solely) by Google
I didn't say anything about other players in the "chat" market.
> Google at some point monopolized the "instant messenger"
No, the have dominance in the email market (I mentioned gmail). They used the position of power in the email market to limit XMPP and support their new Hangouts service.
Just like Microsoft used to do, Google broke their support of the protocol. While most of the protocol still worked, they started dropping auth requests on the floor. This wasn't an error message or missing feature; their S2S protocol simply dropped auth requests so it looked from the outside that the request was delivered, but the person on the other end never responded. Outgoing (from gmail) auth requests were removed entirely. From the perspective of gmail users, XMPP popularity simply faded and Hangouts took over as a replacement.
Using one market (gmail) as leverage in another market (chat) is basically the definition of monopolization. Note that this doesn't require a perfect monopoly in the first market; ability to force/manipulate the market is sufficient. Additionally, the Sherman Antitrust Act also criminalizes the attempt to monopolize.
> Not to mention that XMPP is a standard
Obviously. I've been following the development from the beginning (before it was called "jabber"; the name "XMPP" happened many years later).
> not a competitor.
An open (federated) protocol is the most dangerous kind of competitor to a big company like Google. Deals/contracts can can be made with a company, but a federated protocol by definition cannot be controlled by a single entity.
> This is like saying that modern companies are monopolizing against SOAP or YAML.
I think you need to re-read what I said. Or maybe read more about how antitrust law works.