I'm trying hard to remember if I ever had a 15 minute continuous handshake.
Which is why, IMO, this type of experiment is for purely academic purposes. It was not a forensic study. Nor even an applied study. No one shakes hands for 15 minutes.
Further, of course, such friction on the palms is guaranteed to produce more epithelial cells from which DNA can be analyzed.
Fingertips and palms shed lots of cells, so that we can maintain our exquisite sensitivity in our hands (compared to other primates - all of whom have hands, but none have skin on their hands as delicate as we do).
Anyway, fingers by their very nature (strength is one factor) do leave more DNA on things (doorknobs, etc).
Forensically, shared doorknob data is fairly problematic in forensics, as it is assumed that unless a person lives alone and is the only person touching their knob, it's going to be an admixture. SOME value can be gotten in terms of crime scene analysis, as the major users of the door knob will be known from the study, but as the quoted study shows, anyone shaking hands for 15 minutes before touching the knob will leave DNA from the other person, with whom they shook hands.
Naturally, such a thing never happens in the real world (which is why it is not a forensic study nor an applied study). However, it is useful in showing that if we were to shake hands for 15 minutes, the other person's DNA would be detectable on a door knob newly touched by one of the people. Is the DNA on the knob 50/50 (the study didn't attempt to quantify that - but other studies have, in more naturalistic/applied studies).
That's why the Chief of Police in Moscow was smiling so broadly that day that we later learned was when the DNA results came back from ISL.
SINGLE SOURCE. Only one person had repeatedly used the snap, leaving several layers of DNA in the snap groove. It was not an infinite amount of DNA. I suspect they could probably run one more test - and that they kept at least one swab from the sequencer, so that the Defense could use it if necessary.
Interestingly, the Defense is NOT asking for re-analysis of the sheath. Hmmm.
I bet most of you can guess why not.
IMO.