The story of adulterous woman has also been found in one later manuscript of Luke as well. We don't have the oldest manuscripts or originals, but we do know that manuscripts borrowed from one another in some cases. In the case of the earliest manuscript containing the story we can't say whether or not the story was borrowed from an earlier manuscript we no longer possess (that is older than the earliest surviving manuscripts) or perhaps it was included by an editor because it was such a well-known and powerful event that others witnessed and felt it should be included (all under the guidance of the Holy Spirit). The gospel writers did not and could not include every story and every teaching of Christ in their writing….John mentions this very notion in his gospel. The story of the adulterous woman does not disagree or conflict with anything Christ did or taught...it only enhances it. Does that make it fictional or a forgery? Only in the minds of those looking to oppose Christianity, but given the historical transimission of information from oral to written it’s completely reasonable to conclude the story is 100% rock solid and accuruate. Although, many bibles footnote it’s exclusion in the earliest surviving manuscripts. Further if you include it or exclude it changes nothing about who Christ said he was and the core of the Christ’s message of salvation. For believers the confirmation and validity of its inclusion is found in the Holy Spirit’s guidance; unfortunately this notion will be lost on those that need to understand it the most.