(IANAL)
Well, no, it's not actually up to him just yet. This is a summary judgement--At this stage, he's not trying to prove his case. Instead, during summary judgment the burden of proof is on the party requesting summary judgment (in this case, the guards) to prove that there is no way a jury could reasonably find in the other party's (here, the prisoner's) favor.
Reading the opinion, It really doesn't seem like the guards met their standard of proof. The judge basically says "I believe the prison guard over the prisoners", but that's not the sort of decision you're supposed to make in summary judgement--a judge or jury is only supposed to decide who's more believable after the witnesses have testified in court, where the opposing council has an opportunity to examine their testimony and poke holes in it.
(And I suspect there are a lot of holes in Muraski's testimony. According to the opinion "Muraski also has extensive training in [...] small occult groups..." and when it comes to law enforcement, that's usually equivalent to "Has read Dark Dungeons and watched Mazes and Monsters")