But, if I understand correctly, what HP and SGI did was basically to let Stepanov work on STL while he worked for them, and give some semi-official sanction for the STL releases. They didn't see the STL, decide that they needed to work on it, and set up an independent team to do so.
And, if I understand correctly, Stroustrup saw the STL and said (paraphrased) "Yeah, that's going in."