He's been very careful to avoid calling it a lightsaber regardless. Even then, it's perfectly acceptable under Fair Use (must be transformative, which this incarnation clearly is). So long as Luke Skywalker doesn't show up and hand him a lightsaber, saying, "This lightsaber belonged to my father, Anakin Skywalker. You probably know him as Darth Vader," and then flies off in an X-Wing, he's good.
Anywho, Butters's lightsaber isn't actually a lightsaber, it's a holy sword with one of the Nails from the Crucifixion that was broken and now manifests as a beam of light. I wish Disney all the luck in the world if they want to try to argue this particular case; I give it five minutes before a judge dismisses it.