No, it would be one channel per person. Only you would be able to talk (post) in your channel. People who want to read your posts would join your channel.

Private DMs easily map to one-on-one privmsgs. Public replies and threads won't have an equivalent unless there was some meta-protocol to temporarily allow another person to talk in your channel + duplicate your posts in your channel to theirs + expire that access eventually.

Also subscribing to tags won't work without a similar "copy all messages from one channel to another" relay.

Channels are basically tags, without history for those who join. Anyone can PRIVMSG a channel by default, it is prevented by setting the common +n mode.

My point is that IRC messages need to be duplicated across multiple channels to be the equivalent to how Twitter works.

Well to compare that to Twitter you'd also have to have everyone in the world on Twitter which is overwhelmingly not the case.

That's basically what twitter is.

Make all channels -n and you now have Twitter hashtags, kind of.