The 'special treatment' isn't technical, it's procedural -- insofar as if, during development for a new release, MSVC were to land some changes that broke GSL, Microsoft's testing would catch that and ensure that the changes were reverted or fixed to support the latter, prior to shipping. Since they're built as part of the same operating system, they can make sure not to step on one another's toes -- which is not a guarantee that they can make to third-party applications.