It is not the pen, it is the pen tip. Ballpoint pen tips are microscopic tungsten carbide ball held inside ultra-thin steel sockets. So you need cutting tolerances precise to 0.001 millimeters. If the socket is a fraction of a micron too loose, the ink leaks. Too tight, and the pen won't write.
Source from al-Arabiya: https://english.alarabiya.net/variety/2017/01/14/At-last-Chi...
The point (no pun intended) is that China was beginning to crack the processes for making the precision machine tools that make machine tools.