Are you talking of https://www.aaj.tv/english/national/the-story-of-pakistan-navy-submarine-hangor-which-changed-the-course-of-1971-war-at-sea/ as the source or some other one.
There was no need to execute any attack on Karachi because Karachi was not operational. Read the CIA report I have linked to. That is a neutral source. Your source is Pakistani.