I posted this below, but:
The complaint is not that YikYak wasn't blocked -- it's that no action was taken by the University to investigate the threats properly and readily, even so much as just creating a strong stance on "hey, don't tell people you're going to kill them". Blocking YikYak was simply __an option__ which the University could have taken, but that wasn't the thrust of the plaintiffs' argument at all, nor the Court's Opinion. The point of blocking YikYak was more of a point that "well, you absolutely could have done something", rather than a prescribed course of action.
You're arguing a point that was not made by the plaintiffs and also is not related to the actual court Opinion either. The crux of the complaint is that in light of real, credible threats, the University took no action whatsoever. The Opinion is more about how the defenses of anonymity made it "impossible" for the University to act and that 1st Amendment protections prohibited [the university] from doing so were considered invalid in light of other case law.