If they had no contact they aren't the killer!
If the foreign DNA doesn't match those who were in contact, then it is most likely an artifact. No killer there!
Yes, it should be a short list but the short list is a subset of a larger list.
The short list is made up of those known to be in recent contact with the victim leading up to her death and after her death leading up to the autopsy. Most of this list has been tested and not matched. This simply means that the source is someone not on that list.
The larger list is those in Boulder and surrounding area on the night of the crime. How many of those people are in the DNA database? I would venture, not many. Tens of thousands of persons in Boulder and surrounding area on the night of the crime means that there are tens of thousands of potential matches who have never had their DNA submitted to the database and who will never have their DNA submitted to the database. The foreign DNA found on Jonbenet belongs to one of those tens of thousands.