You can say that..but it was a problem. It was not trivial, despite what the spec says or what is claimed now. If it was, more people would have used it.
I don't know what you mean "is a problem"; what's a problem, setting up groupchat? Everything uses multi-user chat and it's built in to all the servers I know of (but can generally be run as a standalone server if you prefer, which is what this means)? You can just enable it in your config and it just works on at least Ejabberd and Prosody, probably others.