The fact that it needs a bridge is what makes it not federated. Federation allows for interconnection without bridges, because everything talks the same protocol. You can bridge Discord to Matrix, and that doesn't mean Discord is federated.
The reason is because it's just more corporate sloth that has fake federation. If it was even remotely realistic to run a Federated service people would be by the dozens because they have way more users than we do. The only working Federate ironically the activity Pub Bridge