Let us consider a matrix four possibilities. You will have to reformat this table in your word processing program.
RS cut MK cooking RS did not cut MK cooking
MK DNA is present on blade (1) (3)
MK DNA is not present (2) (4)
Let us say (1) that MK did get cut while cooking with RS, and there is DNA present. Then there would be an innocent explanation for the DNA, and nothing ties it to the crime. Now let us say (2) that she did get cut, but no DNA is present. This would be ascribed to someone’s cleaning the knife, but nothing would tie the knife to the crime. Now let us say (3) that she did get cut, and DNA was present. This would favor the prosecution’s case. Finally let us say (4) she did not get cut, and no DNA is present. Thus nothing would tie the knife to the crime.
My interpretation of the defense’s position is that the prosecution’s claim of DNA being present is dubious, and even if the DNA were there, an innocent explanation for it exists. I do not see how the second half of this argument in any way invalidates the first. I would point out that if Sollecito is not telling the truth, possibilities (3) and (4), it does not necessarily follow that he is lying; the alternative is that he sincerely believes something that is not true (I call this a false memory). The distinction between RS’s possibly speaking falsely and lying is important to make from the point of view of language; whether or not it is important to the case is in the eye of the beholder, IMO.
Suppose a different situation existed: Kercher’s DNA is indisputably found on the knife. Sollecito says that neither he nor the other defendants killed her, and she never cooked with him. We would not say that his words exclude the knife as the murder weapon. Why would we say that his words identify the knife as the weapon in this case? BobTheDonkey attempted to answer this question once, but I found his argument unconvincing.
Finally, let us not forget that the kitchen knife does not match all of the wounds, nor does it match the knife outline on the sheet.
Chris