Shallan has latent radiant abilities however, so this could play a role in it. Also, it could be that she actually created the shardblade via another means, but she believes it was made as a result of her killing her father. Also, if you think about the theme of the rediscovery of magic that Brandon is trying to include, someone is going to rediscover how to make shardblades. If Shallan did make hers, then she would provide a great starting point.
As for it simply being a "forgotten" shardblade, that's a possibility, but just owning a shardblade is a great symbol of prestige and even those who hide theirs (king of Jah Keved) still can't suppress the rumors. Although her family was no more than backwater nobility, her father's rising position must have put them under a much greater deal of scrutiny and rumors would get out (plus letting rumors out is sometimes beneficial if you need more clout).
I haven't convinced myself regarding this, but I don't think that we can rule out Shallan making her shardblade.