[Culhwch]

On what evidence? To be honest that thought never occured to me - I think it instead showed the genuine affection that Igraine developed for Derfel and his story, despite the fact it didn't meet her expectations. It seems to me Cornwell intended these books as a lost text found, rather than an original text altered. That the tales were already warped is probably the point here - they are the source of the literature to follow, not Derfel's text. Instead perhaps it was put aside, dismissed as the ramblings of an old man, or else dismissed as not exciting or mythical enough...