Yes, instances can defederate with each other. lemmy.ml defederated with lemmynsfw.com, which is why you don't see these posts.
You can try my instance, discuss.tchncs.de, as far as I see, they federate with lemmynsfw.com.
Do refrain from joining big instances, like lemmy.world, as they are currently dominating the Lemmyverse.
No, I don't think it is an easter egg. Probably just a missing check in code that needed to be done.