So eyewitness testimony is only evidence if it's given in the context of an adversarial court system?
For the purposes of accusing specific individuals of crimes yes, pretty much
But we're not talking about accusing specific individuals of crimes. We're talking about whether (a) there is evidence that the Pinochet regime tortured people, regardless of which individuals were involved in or aware of the torture (b) whether the doctor who was originally mentioned was a victim of torture at the hands of said regime, regardless of which individuals were involved in or aware of said torture.
You're obviously not a fan of the historical method.
That's fine for writing books on long dead history. If a historian wrongly accuses some long dead king of some crime it doesn't really matter, If some ancient purported original source turns out to have been faked centuries later its just an academic question. I'd suggest, for example, that historians know a lot less about pre-modern history than they they think they 'know'
Actually, it's the historians who are most likely to be aware of the limits of what is known about pre-modern history (and, indeed, modern history). And not every question of ancient history is purely academic. There are major world religions whose central claims are based on historical events. There are national and ethnic identities that rest on pre-modern historical narratives. Investigating the historical evidence for and against these things potentially has massive implications for the people who are part of the relevant communities.