Bootstrap is a CSS framework and the flexbox layout is a CSS module, so you can't really compare them.
Do they not provide comparable functionality?
It's analogous to asking if jQuery provides comparable functionality to JavaScript ES5 enumerables. It does, it's just a strange comparison given that the former is a library built on top of the latter language, using a different API due to the former predating the spec
Honestly a Bootstrap / Semantic UI style system that implemented Flexbox would probably be the best thing for making Flexbox more popular.
Older browsers dying so we can actually use Flexbox on sites that have to work back a couple versions will probably help even more.
Didn't seem to work that well for the "Pure CSS" library that Yahoo made, which uses Flexbox.