If it was a major plot twist I'd agree, I'd never spoil "Citizen Kane" the way a Peanuts cartoon strip spoiled it for me, but it wasn't a major surprise or a twist, it was a piece of character development at the end of the series. Even keeping on the safe side, I'd say it's no more a spoiler after all this time than "Dream of the Endless dies in the last storyarc of Gaiman's Sandman"