Sure enough trying to browse to those lemmy.world communities from lemm.ee, they would show no posts.
However, if I switched back to lemmy.world and tried to view those same few communities, I would see posts.
In case there was any doubt about the instances being federated, I was able to see many other lemmy.world communities from lemm.ee, just not these few.
With just one exception, those communities posts are all visible now.
So, idk if this is correct or not but it's what I've heard from others. When you visit a community from an instance that isn't it's "home" instance, it will appear to have no content, if nobody else in your current instance has subscribed to it. The act of subscribing to that community causes your instance to sync its content. After that happens, anybody in your instance will be able to see its content, it will appear in "all", etc. Afaik there's no "master list" of instances and their related communities, so you have to subscribe to a community for the instance to become "aware" of it, so to speak