Pretty sure it doesn’t “take a while” for things to sync up. Once an instance is discovered by another, any posts from that point forward will appear, but anything made before then will not. I have personally seen new instances sync with kbin.social that have less comments than if you look at the main instance’s thread. They have added more comments, but only new ones made after it was discovered.

Whether I’m right or not, I’m not sure. Would have to hear from the devs to know for sure, but from all I’ve seen there is no syncing of archived or historical postings, just discovery of and acceptance of new data from other instances.