The source is mentioned in the OP - PShamim, who is a highly respected member on this forum and pakdef.
A lot of what he has revealed over the past years has been extremely accurate. I will leave it up to you to contact him to determine why he is credible.
I would not have moved that post over and created a new thread if the source was not credible.
That said, even PS has indicated that the acquisition of a nuclear powered submarine is a 'possibility', not a certainty. Penguin has made some good arguments as to why it would not be a good acquisition for the PN.
What is clear now is that negotiations over a potential nuclear sub acquisition have being going on for some time. The specifics of what the PN might be looking for in such an acquisition - systems, specifications, upgrades - may indeed prove to be a roadblock, but we will have to wait and see.