Mujaahid said something interesting. He said that when 9:80 was revealed, the Prophet (PBUH) said, "I will ask more than seventy times!" Mujaahid therefore believes that 63:6 came to put an end to any misunderstanding about 9:80. That is explanation, not abrogation and it appears that Mujaahid so understood. I concur.
Dr. Mustafa Zayd wrote the details of this narration, in his book النسخ في القرآن الكريم, volume 2, pages 230-240 (items 1094-1097). When Abdullah ibn Ubayy ibn Salool died, the prophet (PBUH) was invited to pray on him (the funeral prayer). When Umar saw that the Prophet (PBUH) was about to go ahead and pray on this hypocrite, he jumped over to him and reminded him of what ibn Salool had said. The Prophet(PBUH) smiled and said, "I've been given a choice and I chose. If I know that if I asked more than seventy times, he would be forgiven, I'd ask it." The Prophet (PBUH) prayed on ibn Salool and left. Shortly thereafter, verse 9:84 was revealed.
Dr. Zayd says that this narration is authentic. This means that the understanding that 9:80 offered a choice was the Prophet's (PBUH). He had an enormous capacity for forgiveness and an almost desperate desire that his people escape punishment. That is why, it seems to me, he hanged on this unlikely interpretation of 9:80. But God decisively corrected it with 9:84, and the Prophet (PBUH) never again prayed the funeral prayer on hypocrites.
One might ask, "why then didn't God reveal 9:84 at the same time, so that this matter is unambiguous from the beginning?" My humble answer is that God wanted what happened to happen, so that we all know how forgiving the Prophet (PBUH) was and how undeserving of forgiveness the hypocrites are! It is also an exercise in proper interpretation; we should not interpret verses beyond their obvious meaning just because we wish our interpretation be true.