I think this may be open to different interpretations?
I interpret this to mean that the sample tested was single source, not that it was the only DNA found on the sheath. IMO.
I recall this finding distinctly and the single source DNA of a male was from under the snap of the sheath.
At no point has it been declared by any form of authority that there was no other DNA on the sheath. In fact it would be literally impossible for there to be no other DNA, most especially that of the victim whose body it was found underneath if that information is correct.