When I was working with ejabberd clusters servicing mobile clients it worked but I've had a lot less pain just using a custom protocol over websockets. My personal experience also seems to be mirrored throughout the industry as all the big players eventually move away from xmpp.
GTalk was awesome while it was federated.
Riot Games is also using it for the messaging part of League of Legends (https://engineering.riotgames.com/news/chat-service-architec...)
XMPP is also used by military organisations such as NATO (https://www.isode.com/solutions/military-xmpp.html)
The sad truth about XMPP is that it just works, but it has a very bad publicity because there's just no marketing around it, and it's never used directly by users. So it suffers from that wrong image that is just completely outdated.
That sad truth is that people who actually use it end up dropping it. Its not some conspiracy. It just wasn't cutting it.
You also misunderstand what this api is referring to. These are APIs for your server to connect to Google's. This is not a direct http and xmpp connection to mobile devices. That's all abstracted away from this.
Don't push your words on other people :(