> I also don't understand what it means to "interpret" Swift to Swift. Do they mean they compile Swift to Swift (or the more modern "transpile" which means the same thing)? But it sounds like they're doing something dynamically at runtime, so it sounds more like decompiling machine code back to Swift, but the rest of the post doesn't match that interpretation.

My understanding is that this system walks the syntax, creates autogenerated wrappers to API structures/functions, and simply invokes them as the syntax directs – as it says, a "glorified FFI". It is not a full free-standing Swift runtime, but it's a very clever hack.