They're not though. The author worked for Twitter when he released the first version of Bootstrap, but Twitter never actually used it in production. Twitter doesn't promote it anymore either; BS3 got moved to twbs/bootstrap on GitHub, and the new marketing site doesn't call it Twitter Bootstrap anymore.

ZURB doesn't have a huge consumer product, but they're a pretty big name in design, and they have half a dozen people who regularly work on Foundation/Foundation support.

I agree that I should have written it past-tense... But, until recently, it was clear that Twitter's name was attached to the project. A Google search for "Twitter Bootstrap" pulls up all the relevant docs, and the first line of the 2.3.2 docs is "Built at Twitter by @mdo and @fat"