It brings to mind a certain tongue-in-cheek idiom, which sums up this fallacy rather well:
"If my auntie had testicles, she'd be my uncle"
What Marasca was saying was that HAD the investigation been done properly and professionally, then there MIGHT (
theoretically) have been the means to prove the guilt of Knox and/or Sollecito - if they had indeed participated.
But likewise, had the investigation been done properly and professionally, they it might have been possible to prove that Guede acted alone. Or it might have been possible to prove that Guede acted in concert with a particular Italian drug-dealing associate of his. And so on. And so on.
The investigation was so horribly botched that it was only possible to come to one safe conclusion: that Guede definitely participated in the murder. And thanks predominantly to the gross incompetence and jaw-dropping errors/omissions/negligence by police and prosecutors, it will forever be impossible to know with either judicial or factual certainty (short of a full, reliable confession by Guede to being the sole killer) exactly who else might or might not have been involved.
But this much is true and will forever be true: there's no evidence whatsoever (i.e. reliable, credible evidence) that Knox or Sollecito ever participated in the Kercher murder. And since at the same time 1) there is proof beyond a reasonable doubt of Guede's involvement in the murder; 2) all the evidence (i.e. reliable, credible evidence) is entirely compatible with Guede acting alone; and 3) this sort of crime (sole male burglar is confronted unexpectedly by female home dweller, burglar - in mixture of panic, fear, heightened adrenaline, urges to control, and sexual & sexual violence impulses - attacks, sexually assaults and kills female) is comparatively far more common than pretty much any other group-attacker scenario; it's reasonable IMO to arrive at a working conclusion that this horrific crime was committed by Guede, acting alone.