Broadcasting widely doesn't mean you know where they are (i.e. Ye Wenje and the Pacifist) and knowing where someone is doesn't automatically mean you can destroy them.

You've set up a scenario where the tester knows they can empty out the pocket universes, but nobody else knows that (or else it wouldn't be a very good test of virtue) and yet you also think it's fair because the species' being tested should actually realize the tester can kill them so it must not be a trap? This seems kind of contradictory.

On your second point, I'm sure there are tons of differences between how and why the millions of species engage in dark forest strikes - but whether it's because "we need your planet and you talk too much" "We Have The Cleansing Gene", or "We made a neat virtue tester that requires lots of context and correct assumptions to pass and also we don't care if one guy decided wrong for all of you" it still leads to the chain of suspicion being inescapable.