Sorry for not being clear enough (my fault, even in my native language I am not clear most of the time), but my point was they are not using transports like openfire does.
Look at meebo.com as a libpurple client (just like Pidgin en Adium). After you signed up at meebo, you get an account at the xmpp-server located at meebo.org (your jid will be: firstname.lastname@example.org). Adding a secondary account (msn/aim/xmpp/etc) will connect you via the webclient at meebo.com and not via a transport located at meebo.ORG.
You can do the same with a desktop client like Pidgin for example. You can configure pidgin to login to msn and xmpp, but the msn messages are then not being send via a transport on the xmpp server, but directly to the msn server.
In short: Meebo doesn’t use a xmpp-server for logging into msn/aim/icq/xmpp, but a libary called libpurple. However, signing up does create a xmpp account at meebo.org.
Hope this makes it a little bit more clear .