I mean, they settled as soon as they found this, as they knew how it would've played out in a trial. If you can show a judge that evidence was deleted, the judge/jury would need to assume that the missing evidence would show the guilt of that party.

The needle in the haystack part was that Vivendi were trying to bury Valve in evidence, as they assumed that they wouldn't be able to afford to hire a bunch of Korean paralegals to go through everything. A wealthy opponent will do that in order to put you in the position where you have to choose between settling or paying a fortune to go through the evidence that may or may not contain anything useful.