That's because it's no longer Twitter Bootstrap. @mdo works at Github, @fat at Medium.

We worked closely with the bootstrap community to move the project to an independent home as part of the 3.0 release, you can read about it here: https://blog.twitter.com/2013/bootstrap-30

That list is only for twitter's github repos, bootstrap has been moved to its own github organization[0].

[0]: https://github.com/twbs