If you wrote an MPSC queue (standard question) with multithreaded demo in 20 minutes in C++ you’re pretty hot shit, mate. Their loss. It’s not that it’s hard. But that speed without error is just really good. C++ is particularly unforgiving too.

He didn't get the job so changes are it wasn't correct.

I can't remember the exact problem or how long it took but it was definitely some awkward multithreading. I'd rate my C++ as pretty good but probably not hot shit!