Not every character that has Balmung as a home world is online at the same time. Most of the time, the world server is fine handling those actually logged in with room to accommodate visiting players.
If Balmung did hit concurrent online capacity, genuine queues to log in to Balmung would start up (not the usual 50-100 player queues that last all of a minute that we normally see but those 5000+ player queues that would last 4-6 hours we saw at the start of Endwalker). SE would also have the ability to lock Balmung to data center travel and world visit if necessary.
The Congested status works more to redirect new character population elsewhere so Balmung is unlikely to hit that concurrent player limit and to help the log-in servers function better. If I remember correctly, characters in the process of being created are using up log-in server space. If they can't be created, then the log in servers only need to process those actually trying to log in.
I see more club spam for Aether and Primal venues than I do for Crystal venues but then I don't live in Limsa so it could just be the timing of when I'm passing through.