They are starving because of the massive effort put into other things.
It is 72 years since V2 and Bomb #1, lots of engineering that was incredibly difficult in 1945 is absolutely routine today, even in N.K.. Lots, not all, of the physics is now known to anyone with a good university education. Doubtless N.K. will have taken help where it could be found but it isn't beyond reason to think they could have done it all themselves.