All IRC messages are simple strings in the augmented BNF representation.
He does not know what a formal language is.
The difference between a command and a message is just the prefix that you type in the client. Any message that you type beginning with the "/" character is interpreted as a command.
He is confusing the IRC protocol with a user interface now.
IRC messages are short because they are simple ASCII text strings in a specific format.
First of all, encoding issues are a major pain point in IRC. Luckily most people use UTF-8 now, but the transition from legacy 8-bit encodings was not easy. Considering the history of IRC, I don't believe that IRC ever used plain ASCII.
Secondly, what is XML but "text strings in a specific format"? That's incredibly vague statement to begin with.
There is no difference between commands and messages in IRC, because to send a message, the command PRIVMSG  is invoked:
PRIVMSG #channel :Hello interwebs!
MSN/AIM/qq/XMPP/etc <-> bitlbee <-> your irc client
It's the bee's knees.
This post looks a bit like filler, it's not very accurate on the details (as other posters have said), and it doesn't have much content.
I think the main reason people will prefer IRC over XMPP is one of culture. IRC is for talking to strangers, XMPP is for talking to people you know.
Because of this, various company chat services are gearing towards supporting XMPP, because people usually already use XMPP clients, and the whole environment is less "let's look around the server and see what channels are interesting" and more "I'll just make a channel here because everyone has a client that can talk to it already.".
Both technologies are great, and you can use them pretty much interchangeably, but each serves a different target market. That's the biggest difference, in my opinion.
Instabot sounds good. I'll be happy to try it out with the Gaglers chat rooms. There are not much details on the site about how to customise the bot.
Can I email you to ask what sort of customization you're looking for? Thanks!