That's not true. A single person can do it at world scale provided they are not trying to do something overly esoteric (though you can actually get pretty far with a Gentoo base). There are even plenty of from-scratch distros that started as a one-man show. Take the recent Void Linux for instance.
In a dictature killing access to culture and education, it not weird to assume it is hard to breed those so you have a limited pool.
So yes, I understand the surprise. Which means NK has some resources we don't include in the picture.