Citation from the article:
Traditionally when two languages need to interoperate, the function calls at the boundary between the two languages, also known as the Foreign Function Interface (FFI), will go through C using a library like libffi. This approach has some drawbacks such as incurred runtime performance costs and possibly extra boilerplate code. Instead, Swift embeds a copy of clang, the C and C++ compiler, which is able to directly translate between the languages avoiding penalties in code size and runtime performance.
And this goes this way (citing from above):
Those header modules are also mentioned in the article.
And this is what I meant about Microsoft being the gatekeeper to this kind of native development.