Seeing as how broken Stoick was after Valka’s “death” I don’t think it’s something he and others spoke very openly about, so I’d like to think that Hiccup’s friends really only had really vague knowledge of what actually happened to his mom.
Like, she died when Hiccup was a baby and the chief blames himself, they know nothing more than that.
So just imagine on day, Hiccup, who in his teenage years became comfortable talking about the subject, goes: “yeah and my mom was eaten by dragons”