I don’t think of the threadiverse as a link aggregation platform but as a network of communities engaging in threaded discussion. The federated model is an answer to the problem of platform lock-in, the network effect, and the lack of autonomy communities have on proprietary/commercial/centralised platforms.
Each instance separately may fill the role of link aggregator but mainly for that community (instance), with that community’s values and moderation policies. The ability for an instance to federate with other instances with compatible policies is the benefit here.
It may actually help if you view an instance as the community, with its “communities” as its topics.
I never used Discord but used google hangouts before switching to Telegram and Matrix (the former for family and the latter for everything else).