Hi Gideon, it's an interesting question but I'm not sure that I quite understand what you're asking (I'm not an academic philosopher though, so I may be misinterpreting what kind of answer you're looking for)?

Like, why would a statement about existential catastrophe have different truth/meaning to other statements like "The earth revolves around the sun"/"Slavery is morally wrong"/"Arsenal are the most likely team to win the Premier League this year"? It sounds like you're asking about theories of truth and Philosophy of Language, rather than about xRisk?