You’re very sure because you’ve built one at scale? This is such peak HN. And you have plenty of other people who have never built a billing system at scale to agree with you. The OP is right, this is in fact very very hard for the reasons they say.
The idea that most people who are doing professional public speaking are reliable narrators is a bit quaint. There is a lot of room for framing that you have to allow for story telling. If you think that all story telling is about reliable narration, you're going to have a tough time being successful at it or interacting with others who are.