Where'd you hear about a second season? I can't find any news about this.
Yeah all that's been confirmed so far is the Goblin's Crown movie which is likely what that endcard was referring to.I think people assumed it was happening because of the "Goblin Slayer will return" line at the end or maybe because Funi decided to stick season 1 on the box. All that's been announced so far is the Goblins Crown Episode?/OVA?/Movie?
Yeah I can see why you'd say that but it's very likely just referring to the movie. At least it's not as bad as Drifters which explicitly announced a second season before continuing the story as an OVA series. Goblin Slayer has made no such promises.This is exactly the reason I assumed there would be more.
I believe the anime only covered the first light novel volume