The Norks were probably able to get a sample Chinese engine of this type to reverse engineer and then made exact copies because that is all they had the resources to do
More power than you need is a nice problem to have when trying to deploy big, heavy 1940s and 1950s style low tech nuclear warheads on long range ICBMs
That was my reaction as well. This "analysis" assumes NK would be launching compact warheads developed in the 70's and 80's, instead of a large heavy one reminiscent of the early atomic age.