You have it backwards. The USA was a Confederation (a weak alliance of almost entirely independent nations) under the Articles of Confederation, 1776-1789
Then in 1789 it became a Federation (a strong national union of autonomous but not independent states) with the new Constitution.
In a confederation, the central government has negligible power and states can come and go as they please. The EU is arguably a confederation, and it's pretty much the only modern example, because confederations rarely last long. Historical confederations include the CSA and Holy Roman Empire.
That is not the model that the Fediverse wants to uphold (otherwise it would be called the confediverse). Mastodon is intended to be structured as a Federation, like the United States, Germany, or Malaysia, with a strong but limited central power that maintains unity among the federated members, while mostly letting them do their own thing.