Most scholars think that it was probably a well-known story circulating in the oral tradition about Jesus, which at some point was added in the margin of a manuscript. From there some scribe or other thought that the marginal note was meant to be part of the text and so inserted it immediately after the account that ends in John 7:52. It is noteworthy that other scribes inserted the account in different locations in the New Testament—some of them after John 21:25, for example, and others, interestingly enough, after Luke 21:38. In any event, whoever wrote the account, it was not John. That
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Considering that most, if not all, material of the Gospels go back to oral tradition, being a story that comes from outside John should not disqualify it from cannonization. Perhaps it should only be disqualified from arguments and debates related to the Gospel itself.
