According to swift.org/migration-guide, dispatch and related functions have a great new swift interface (e.g.
SDK Overlays: Specific to Apple platforms, the SDK overlays (implemented in stdlib/public/SDK) provide Swift-specific additions and modifications to existing Objective-C frameworks to improve their mapping into Swift. In particular, the Foundation overlay provides additional support for interoperability with Objective-C code.
tl;dr anything in these folders are Apple platforms only. Still not sure why they restricted the Dispatch overlays to Apple Platforms since
libdispatch is also open source and targeting linux, but this answers my initial question.
Edit: was able to confirm via swift mailing lists that the dispatch overlays do have some reliance on objective-c runtime. There is work to get them implemented for non-Darwin platforms but no guarantees on timing.