I don't think IRC is a good analogy here because there's no "message resync" that happens when the netsplit is resolved. If there are two people on opposite sides of a split, and they both send messages to a channel while things are still split, they will not see the other's messages when the split is over.
In the Matrix case, if a homeserver disappears for a while, it will sync any missed messages when it comes back online.
In the Matrix case, if a homeserver disappears for a while, it will sync any missed messages when it comes back online.