That’s not how any of this works…

Communities (as you put, sublemmy) can’t be individually federated, only instances can be. And needing another account is a sign that you are looking at a post on the wrong instance, not a sign that an instance is not federated. In fact, finding out an instance is not federated can be pretty difficult unless you check the list of instances that are defederated on another instance.

That’s not a universal link, so you ended up on a different instance. A much simpler and easy explanation than what you came up with.