As always, when making these kind of statements, the question is: relative to what, and for what purpose? Without knowing that, it’s quite hard to know what the author of this page is getting at. Who is he arguing against, and what’s the context for these “myths”?

XMPP is sort of similar to all the browser technologies (HTML/DOM/CSS/etc.): it’s a large and complex spec which takes a large amount of work to implement even a basic version of, with lots of possible things to screw up.

It mixes various concerns together into a big jumble without the cleanest layering, which makes certain types of systems a pain to build on top of it. It’s based on a technology (XML) which is inherently heavyweight and not all that well suited to streaming applications. Tooling which exists around XML is mostly fairly irrelevant to an XMPP-like use case. If someone wanted to make an open IM protocol from scratch, they could make something much simpler, cleaner, and better organized.

On the other hand, all the other popular person-to-person IM protocols are proprietary, and XMPP is at least open, reasonably well documented, and with a relatively large community built up around it. Which counts for a lot.

If someone wants to build a new protocol, for example Google Wave, I would very strongly recommend against trying to shoehorn it into XMPP. On the other hand, if someone wants to make their chat client or server interoperate with existing chat networks, then XMPP is great.