I've done a lot of work on XMPP related projects and some specs, and I'm not sure that this is true. I mean, it's definitely complicated, that part is true, but of course it is, it's a protocol and it's not more complicated than other specs as far as I can see. Not to mention that, like the various clients for different platforms that this article mentions, one of the big benefits of XMPP over other protocols is that it has libraries for every major platform, so most developers will never need to implement the spec.

TL;DR most people don't implement HTTP when they want to make a request, they add a dependency on whatever their languages HTTP library is. XMPP is the same.