They're getting bits of it...... slowly..... even though they don't even know they are
One of the idiots has just (correctly) pointed out that in human blood, there is a vastly higher proportion of red blood cells (which contain no DNA) than white blood cells (which do contain DNA). In fact, in any drop of blood there will typically be around 800-1000 red blood cells present for every white blood cell present (this commentator said 400, but at least he's roughly in the right ballpark.....).
And, as he correctly pointed out, this means that with certain small blood stains, it's not uncommon to be unable to get a positive DNA result, since there may well be very few DNA-containing white blood cells present.
Now.......... I wonder how long it might take this commentator (or any of the other idiots) to figure out the logical companion to this: that if you get a positive DNA test without a positive test for blood, you can be (to all practical purposes) certain that this was not blood DNA (since if there were sufficient white blood cells present to yield a positive DNA result, there should be many thousand red blood cells also present to give a positive test for blood).
And then how long will it take the idiots to apply this new-found piece of scientific deduction to the issue of the DNA on Sollecito's kitchen knife?
I'm not confident the the penny will drop. But you never know