The point is: you interpret words as they'd be understood by a reasonable person in the position of whoever the words were spoken to, not as they were understood by the person who spoke them.
E.g. consider the Hartog case. Key sentence: "...the plaintiff could not reasonably have supposed that that offer contained the offerers' real intention." ("Could not reasonably have supposed" is legal shorthand for "A reasonable person in their position would not have supposed"). If they had genuinely thought that the offeror meant it, it wouldn't have mattered.
No test by fire needed.
(NB this is English common law. I've been told in the past (https://news.ycombinator.com/item?id=5402595) that the UCC in the US is a whole lot more subjective than English law, so this is likely wrong in the US).