John 7:53-8:11

I read an article claiming that John 7:53-8:11 (the woman caught in adultry) was written after the fact and that John did not write this in his original text. There was plenty of evidence to support this and I am inclined to agree with what the author is saying. There was a section in the text that stated, "If John wrote his gospel as a tightly woven argument, with everything meeting a crescendo in the resurrection, would he be disturbed that some scribes started monkeying with his text?" This was the overall problem that was being addressed by the article. As I read this, I was thinking of the lines in Revelation 22:18-9:

"For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and [from] the things which are written in this book."

My thought is this: What if John knew that there were writings being added to his letters and he wrote this in Revelation to wart off any changes? To my knowledge, John did not say anything like this in his previous letters. John wrote his Gospel within 20 years (+/-) AD 70 and Revelation around AD 95. It is very possible that in his edition of the Gospel that he sent out, he received a copy of it back years later that was modified and at the time that he wrote Revelation he added the above warning so that it would not be changed. It would make sense that the warning in Revelation was to stop scribes and other people from changing what he had written. It is also possible that John wanted to send out his original letter again but was not able to due to time constraints. There was also the possibility that if he were to send it out again that nobody would believe that it was the authentic account.