I think he’s saying that if the 10 people witnessed a murder via gunshot that it would be mighty suspicious if the state didn’t bother to bring any forensic evidence of the shooting to court and instead relied solely on those eyewitness testimonies. At some point the absence of forensic evidence might look more like a deliberate attempt to keep exculpatory evidence out of reach of the defendant than a good faith attempt to prosecute.

Perhaps, but that coin has two sides. A lot of people have watched a ton of crime shows and that warps their sense of what "reasonable doubt" is.