The isolation level you speak of is between networks, not between servers. This distinction might seem a little arbitrary since no centralized system of relevance is running on a single server either, but the key difference is still there: in the larger irc networks, the different nodes that form it are run by different organizations.
Splits are a possibility in any federated system and in systems with nonhierarchical, unique naming, any merge requires some amount of namespace violence proportional to the duration of the split.
