Agreed. The only reason that this might not drive down the probability of Knoxs guilt is that there's some chance that the system is behaving somewhat rationally, and the fact that they were convicted is evidence that there's information that you don't have.

Of course, once you hear all the prosecutors arguments, that goes away.

That brings up an interesting question though. If the base rate is low enough and base rate neglect is common enough, maybe you really can confidently claim that Knox is innocent even though she was convicted given only the fact that Guede is guilty and plausibly could have done it himself.