I really don’t see the benefit of forge federation. Why do people care that completely separate projects run in completely separate forges?
A project’s issues and pull requests are only useful for that project.
The author mentions avoiding multiple logins and searching across forges. The former is already addressed by social logins / federated identity. The latter is not very useful today, on a centralized GitHub, aside from finding leaked credentials and vulnerable code.
In fact, the tiny barrier of entry for contributing in a new forge might be a desirable quality, to filter out low-effort contributions.