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.
There’s a greater likelihood that the content creators are the ones moving. Most of the reddit power users likely used third party apps. Most of the reddit power users are also the ones who wrote most of the comments worth reading.
So if on june 1 most of the reddit power users flee, reddit’s enshitification will have reached a terminal stage. Eventually, reddit will stop having things worth reading, and the lurkers will all move over.
I think we’re in for a long decline of reddit a la facebook. However unlike facebook, there isn’t a market of old people/foreign markets that can fill their user numbers.