What they mean is you join an irc server with tons of communities all in one place. Discord requires you to join different "servers" where each switch between communities feels like you're at a totally different irc server (even though technically it's not). On irc you just have one name and that's what everybody knows you by. On discord you have one name per "server". There's not really a lot of cross pollination there.

Personally I would like to see a discord interface where there's not a jarring difference between each server. Something more like this hexchat screenshot (although hexchat itself isn't pretty) https://hexchat.github.io/screenshots.html but note the server and channel list on the left. I want something more like that.