If they really want that market, they can make a half-hearted effort at a BSD-like package with some MS-specific extensions to court scientific developers. They did buy DEC's FORTRAN and market it as "Powerstation" or some such for a while, IIRC.
But I think the supercomputer market is too small for MS to worry about other than for bragging rights. They're probably more interested in FUDding the open OSes. Maybe they'll do an internal project, release some press food about their super results for their "forthcoming" super product, then let it drag out ...
I dunno. But they can seemingly call (almost) anything they like Windows even if it's not.
Cheers,
Scott.