Just because you 'plainly state' there is no way to know, doesn't mean there's no way to know.
There IS a way to know and its right in front of you. The DNA was reported MIXED with blood, and matching touch DNA was also found.
There is one and only one way this could happen, and it requires the DNA be deposited with the blood and the DNA deposited on the longjohns at the crime scene.
You're wrong.