Edit: ok, I think I get it. So if someone from a random instance posts to e.g., technology@beehaw.org, and that instance doesn't know about kbin.social, it won't get pushed to this instances representation of the technology@beehaw.org magazine?
Correct. It's only "pushed once" by the instance that the creator's account is on. Of course it will push to the source of truth, but it will only push to other instances it knows are also following that magazine since it doesn't necessarily know all of the instances that follow that magazine. In your example, yes, if the creator of the content (which in this example is an account not on beehaw.org) posts to technology@beehaw.org, their instance will push to beehaw.org and others, but if it doesn't know that kbin.social has any followers of the magazine/community, it won't push to kbin.social.
One other possibility is that the 3rd-party instance does know about kbin.social (for example), but has blocked (defederated with) kbin.social OR kbins.social has blocked that 3rd-party instance, my expectation would be that such content won't show up on kbin.social's "copy" of the community.
Treedrake