It is problematic for me, at least, if the panty DNA was found to match the longjohns. To me, this rules out DNA left by someone involved in the handling/manufacturing process before the panties were bought by Patsy.
BUT it still does not prove the DNA is the killers or someone who was there when she was killed.
The way I see it, the panty DNA could still be the same skin cells as the longjohns, which became mixed with the liquid drop of JB's blood. The blood wouldn't "liquify" the skin cells, but the skin cells presence in the drop of blood could yield the two profiles from that drop of blood.
I know you are not going to believe me but you bank that the panty DNA matches with the skin cells on the longjohns. That is a given.