Is the PLA really that much behind in sub tech compared to other areas? Behind ground forces maybe, but air force, surface combatants? If anything PLA sub tech has been more advanced compared to other areas, the conventional subs at least are probably the first combatants to reach world class level out of naval and air force systems. That the PLA was able to develop a nuclear sub in the 70's, however poorly it performed, was nothing short of a small miracle, and there were no comparable achievements in the naval or air force arena for decades to come.
I agree that we're all outsiders looking in, which makes it confusing how you're so sure that the PLAN SSNs are so far behind when what little objective data we do have suggests otherwise.