Evans lived in the house, used the bathroom daily, cleaned-up the bathroom (placing the fake nail in the trash), heard accusation, retrieved the nail and presented it to police. So unless the kid was wearing gloves then of course his dna would be on the fake nail.
Cheshire actually made an indirect point that the dna found on the nail was not just on the underside (fill in the blank) and chastised the press for running with the story that the dna was only found on the underside (while pointing out no blood from the "fierce fight" was found).
You don't have to be a forensics expert to know that if one grabs a small object like a fake nail that you have to grab both sides.
Yes, if the DNA was on not on the underside of the nail, it is even less than useless. But also I am saying one would think that it takes a tiny tiny tiny amount of material not to be able to find one cell to get a full profile from.