If that's their justification, I hope they have a solution to Speak with Dead (or did they somehow liquefy all the hobgoblin corpses?), Legend Lore, and all the other spells any mid-level cleric or wizard can use to find out exactly what happened.
(Of course, it's not. "The only good goblins are dead goblins" is the justification. Really, he told us what he is. Why is anyone trying not to believe him?)