What you're missing is that said supercomputer is a unique configuration of a whole lot of commercially-available parts (hardware and software) that were not designed by Chinese engineers.

Sure, they needed new circuit boards, but that's the computing equivalent of a car frame. They bought the engines, transmissions, fuel pumps, etc. from other (mostly American) companies, and then designed a new frame that could fit it all.

This project isn't a demonstration of locally superior engineering. Rather, it's a demonstration of what you can accomplish when you treat computing capacity as an "arms race." This project cost nearly $400M. If you had $800M lying around, you could have a machine that's twice as fast.