It is not OS X, but the Classic Macintosh System translated on the fly to native code. Because it didn't get enough funding, and lack of interest, and very few buyers it sort of went DOA. I am sure if they had enough funding they could have modified it to OS X translation. Maybe someone should help them do a Kickstarter project?
Having worked on a translation layer for a large API (my project Alky converted Windows games to run on Linux and OS X), building something like this solo is just asking for pain. An open source project with solid direction is much more likely to end well. Also, Executor really couldn't be changed to support OS X; totally different architecture, no processor emulation, etc.
At the end of the day, it's really just API translation; not a technically difficult project, just an incredibly intensive one.
They're really not comparable at all. Executor is a 68k emulator + APIs for OS7; the tough part there is doing the emulation properly, and then just implementing all the APIs. Darling is just a binary loader + APIs; the loader is trivial but there are a bajillion APIs. Probably 3-4 orders of magnitude more complexity in Darling, simply due to how much is in OS X.
I do think they're on the right track, though. Hoping to lend a hand when I get some free time.
I remember it well. I still remember the Webcrawler/Yahoo/Altavista search that took me to it. I just had this inkling that it was something that HAD to exist. Sure enough, it did. And the demo version was awesome. Even more awesome was the clean room techniques they used to develop it.