It has an optional fluid grid:

http://twitter.github.com/bootstrap/examples/fluid.html

In any case, fixed width layouts which adapt to screen sizes are also considered a form of responsive design.

(from the linked site) I find the notion that a phrase as general as 'responsive web design' could be considered 'coined' in 2010, and could be considered to have such a narrow meaning (i.e. you can't be 'responsive' without a fluid grid). A search on the phrase from Jan 2000 until Jan 2009 returns more than 16 million results.