I think that the item would have others’ DNAs on it. However, the fact that only BK’s DNA was lifted from the sheath and it was touch DNA is a little bit suspicious. Think of the opposite situation, like leaving your pen at the crime scene. It would contain your DNA, but potentially, these of the people in your household, your work, your bank.
Now, just one spot of touch DNA, and only yours, at the place where the pen is usually held, would be suspicious. It is as if someone bothered to remove other potential sources before lifting yours. Or more likely, cleared all and transferred yours on one spot. It is not difficult to do. When did BK buy the knife on Amazon?
Someone can buy exactly the same later.
Technically, handling of such purchase would assume, the sellers’/packagers’ DNA on the sheath.
Then all DNA is removed, period. Isopropyl alcohol, acetone, any strong solvent.
Any of BK’s things he routinely uses (the collar of his clothes, etc) can be a source of DNA.