> Are derived community works, like say the D-language, Dart, or Zig bindings to the win32 API which are generated from those files open source - if Microsoft did not release them as such?

Certainly. The source for these bindings is the WinMD file, which is MIT licensed. While it is true that its contents are generated wholly from non-FOSS sources, it doesn't impact end users of the metadata. Microsoft owns the original IDL/headers and can choose to license any related work however they like.