I understand how most court decisions are made, e.g. those involving legality (or morality) of some action. E.g. it's clear to me why a court could find it legal/illegal that the CIA had a jail in Poland.
In this case, however, the court is deciding on facts. I guess what happened here, as vidarh's comment implies, is that the court had some indirect evidence presented, and based on that evidence, ruled that it is more likely than not that the CIA had a jail in Poland (and is concealing the real, definite evidence).
But no one pretends that courts decide in the "fact" - they just determine who is "guilty" of murder. In the US, for example, you have to be certain up to the standard of "beyond reasonable doubt", but even that standard, albeit high, means that the court don't establish facts, but only what the fact is most likely to be.
How can you be guilty of murder if you did not, in fact, kill the victim? Absolute certainty is an illusion. The word "fact" is never meant literally by people who've thought about the subject for a while.
In this case, however, the court is deciding on facts. I guess what happened here, as vidarh's comment implies, is that the court had some indirect evidence presented, and based on that evidence, ruled that it is more likely than not that the CIA had a jail in Poland (and is concealing the real, definite evidence).