If Japanese companies stole the source code, then why was BS2000 developed by Siemens (a German company) from 1970 on [1], with Siemens-Fujitsu taking only over in 2000, 11 years after the date in your link?
And yes, the BS2000 uses the /360 ISA, but so did a few other imitators of IBM. And copying an ISA is widespread, and not restricted to the /360.