To me, the STL is specifically Stepanov's idea for how to do generic programming in C++ in the early 1990s, and so it makes sense to distinguish the STL from unrelated stuff that happens to be in the C++ Standard Library.

For example iostream isn't part of Stepanov's idea, Stroustrup created that bundle of craziness in the 1980s and today it's part of the Standard Library, but it was not part of STL which only came into existence in the 1990s (although Stepanov's ideas about generic programming are older).