You don't access any Fediverse project with the same user and I never claimed there is a Fediverse service that Matrix has a OAuth option for, just that it's just OAuth and can be implemented easily to basically everything, there are no shared accounts between Fediverse services and there are barly any Fediverse projects with OAuth implementations between each other ether, Pixelfed recently added Mastodon OAuth but even that's a relatively new feature.