Zimmerman didn't backdoor PGP. He published the source code in a book that you could turn back into digits through scanning it by OCR. You don't seem to know the history of source code as protected first amendment speech in the early 1990s. US export law in the early 1990s regarding 40+ bit encryption being designated as a munition and prohibited from export. Zimmerman's code was always in the open. The code vs munition was ultimately challenged in Bernstein vs US, and Bernstein won.
https:// en.wikipedia.org/wiki/Bernstein_v._United_States