But the character in question saying it is a reputable source
There's more reason to believe he's right (being one of the most knowledgeable characters on magic in the series, and being an immortal who wants to die) when he says it can't kill him over the idea that he's wrong and it can kill him.
like damn this isn't some random saying this
it's practically the author's mouth piece
Okay so then Touma can be killed by guns, damn everything else he has done or the forms he goes through then? Because that's precisely what you are saying there.