I believe this explanation works on hand DNA, fingernail DNA, or face DNA, but I feel we're forgetting that this DNA was found in the inside crotch of JBR's underwear, an item known to be handled by a criminal.
It also matches the DNA on the longjohns, something known to be handled by her parents. If they got those skin cells on their own hands and it transferred to the longjohns, it could have gotten on the panty crotch when they put them on her.