But I know a lot of people still run Windows because they want the Windows version of an app, either because it isn't available at all on Mac or just because the Windows version runs better (Excel was a classic example of this for a long time, might still be). In that case, I don't know if that same translation layer will have the same performance (if it can run at all outside of MacOS) when running an entirely different OS.
I think this is called "transpiling" -- a version of compiling that's mainly translating from another architecture. And it didn't sound from their description like it was JIT -- it sounded like it would do the transpile when you first installed it (or maybe first ran it?) and keep the results.
And they have a first pass AOT, with a JIT backup from the sounds of it to support JITs like browsers, node, and java.