That is not taking in to account BK cleaning the sheath then wearing gloves for all handling there on.
So it also stands reasonable that there were only that much
after the cleaning of the sheath.
From your link the study states that it is more probable to obtain a mixed DNA if there has been a secondary transfer.
There is no mixed DNA like we see in other cases where there has been secondary transfer.
Perhaps BK is one of your linked studies’ low shedders that naturally doesn’t leave much behind.
>>
Progress in DNA profiling techniques has made it possible to detect even the minimum amount of DNA at a crime scene (i.e.,
a complete DNA profile can be produced using as little as 100 pg of DNA, equivalent to only 15–20 human cells), leading to new defense strategies.
>>
Further studies clarified that on ‘shedder status’, not two but three categories of status should be used: high, intermediate, and
low shedder
3.2. Main Findings
Based on their results, the authors concluded that secondary transfer should be considered a very unusual event.
4. Discussion
Based on these findings, DNA transfer remains challenging in forensic science, both in case evaluations and in court testimony. Considering the results of this review that show the problems
related to indirect transfer, it is more probable to obtain a DNA mixture from a piece of evidence.
Indirect DNA Transfer and Forensic Implications: A Literature Review - PMC
All imo