I'd mostly attribute that to the MKL and their ability to just have to deal with their own instructions. But that's just an "educated guess".

Probably also having full time people working in the same company as the hardware guys who's job is to make the hardware look good.

I understand the social reasons, but I was wondering more about what compiler does in terms of technical achievements that goes beyond what gcc/clang can successfully generate. Surely this must be something that can be studied empirically.